Merge with changes that implement config file PR #15548.

Field *EnsApi string* in swarm/api.Config is replaced with
*EnsAPIs []string*.

A new field *EnsDisabled bool* is added to swarm/api.Config for
easy way to disable ENS resolving with config file.

Signature of function swarm.NewSwarm is changed and simplified.

This commit adds a TOML configuration option to swarm. It reuses
the TOML configuration structure used in geth with swarm
customized items.

The commit:

* Adds a "dumpconfig" command to the swarm executable which
  allows printing the (default) configuration to stdout, which
  then can be redirected to a file in order to customize it.
* Adds a "--config <file>" option to the swarm executable which will
  allow to load a configuration file in TOML format from the
  specified location in order to initialize the Swarm node The
  override priorities are like follows: environment variables
  override command line arguments override config file override
  default config.

With this change,

    key, err := crypto.HexToECDSA("000000...")
    
returns nil key and an error instead of a non-nil key with nil X
and Y inside. Issue found by @guidovranken.

Issue found by @guidovranken

Now that the AES salt has been moved to the payload, padding must
be adjusted to hide it, lest an attacker guesses that the packet
uses symmetric encryption.

As per EIP-627, the salt for symmetric encryption is now
part of the payload. This commit does that.

Also remove installation steps for fuse and golang.org/x/tools/cmd/cover 
because they're not required anymore.

Fixes #15196

We need those operations for p2p/enr.

Also upgrade github.com/btcsuite/btcd/btcec to the latest version
and improve BenchmarkSha3. The benchmark printed extra output 
that confused tools like benchstat and ignored N.

Also unexport isHex, hasHexPrefix because IsHexAddress is the only caller.

Fixes #15550

Allow multiple --ens-api flags to be specified with value format
[tld:][contract-addr@]url.

Backward compatibility with only one --ens-api flag and --ens-addr
flag is preserved and conflict cases are handled:

 - multiple --ens-api with --ens-addr returns an error

 - single --ens-api with contract address and --ens-addr with
   different contract address returns an error

Previously implemented --ens-endpoint is removed. Its functionality
is replaced with multiple --ens-api flags.

Generator in the current lib uses -2 as the y point when doing
ScalarBaseMult, this makes it so that points/signatures generated
from libs like py_ecc don't match/validate as pretty much all
other libs (including libsnark) have (1, 2) as the standard
generator.

This does not affect consensus as the generator is never used in
the VM, points are always explicitly defined and there is not
ScalarBaseMult op - it only makes it so that doing "import
github.com/ethereum/go-ethereum/crypto/bn256" doesn't generate
bad points in userland tools.

Updated use of Parallel and added some subtests to help isolate
them. Increased timeout in RequestHeadersByNumber so it
doesn't time out and causes other tests to break.

p2p/simulations: introduce dialBan

- Refactor simulations/network connection getters to support
  avoiding simultaneous dials between two peers If two peers dial
  simultaneously, the connection will be dropped to help avoid
  that, we essentially lock the connection object with a
  timestamp which serves as a ban on dialing for a period of time
  (dialBanTimeout).

- The connection getter InitConn can be wrapped and passed to the
  nodes via adapters.NodeConfig#Reachable field and then used by
  the respective services when they initiate connections. This
  massively stablise the emerging connectivity when running with
  hundreds of nodes bootstrapping a network.

p2p: add Inbound public method to p2p.Peer

p2p/simulations: Add server id to logs to support debugging
in-memory network simulations when multiple peers are logging.

p2p: SetupConn now returns error. The dialer checks the error and
only calls resolve if the actual TCP dial fails.

Implement a CLI flag that can be repeated to allow multiple ENS
resolvers for different TLDs.

common: fix hex utils to handle 1 byte address conversions

* core/vm: track 63/64 call gas off stack

Gas calculations in gasCall* relayed the available gas for calls by
replacing it on the stack. This lead to inconsistent traces, which we
papered over by copying the pre-execution stack in trace mode.

This change relays available gas using a temporary variable, off the
stack, and allows removing the weird copy.

* core/vm: remove stackCopy

* core/vm: pop call gas into pool

* core/vm: to -> addr

cmd/utils: bootstrap nodes in config file were not respected

* trie: make fullnode children hash calculation concurrently

* trie: thread out only on topmost fullnode

* trie: clean up full node children hash calculation

* trie: minor code fixups