Fixes #3762.  Details about parameter:
https://github.com/rs/cors/blob/a62a804a8a009876ca59105f7899938a1349f4b3/cors.go#L50-L54

* common/math: optimize PaddedBigBytes, use it more

name              old time/op    new time/op    delta
PaddedBigBytes-8    71.1ns ± 5%    46.1ns ± 1%  -35.15%  (p=0.000 n=20+19)

name              old alloc/op   new alloc/op   delta
PaddedBigBytes-8     48.0B ± 0%     32.0B ± 0%  -33.33%  (p=0.000 n=20+20)

* all: unify big.Int zero checks

Various checks were in use. This commit replaces them all with Int.Sign,
which is cheaper and less code.

eg templates:

    func before(x *big.Int) bool { return x.BitLen() == 0 }
    func after(x *big.Int) bool  { return x.Sign() == 0 }

    func before(x *big.Int) bool { return x.BitLen() > 0 }
    func after(x *big.Int) bool  { return x.Sign() != 0 }

    func before(x *big.Int) int { return x.Cmp(common.Big0) }
    func after(x *big.Int) int  { return x.Sign() }

* common/math, crypto/secp256k1: make ReadBits public in package math

Also tweak behaviour so colors are only enabled when stderr is a terminal.

* swarm/network: integrity on incoming known chunks
* swarm/network: fix integrity check for incoming chunks
* swarm/storage: imrpoved integrity checking on chunks
* dbstore panics on corrupt chunk entry an prompts user to run cleandb
* memstore adds logging for garbage collection
* dbstore refactor item delete. correct partial deletes in Get
* cmd/swarm: added cleandb subcommand

closes #3650

fixes #3444
fixes #3494
networkid override

Added comments to explain why test against 0 appears twice

* Command line overrides saved config, saved config overrides system default

---

fixes #3476
bzzr get with path

Finally a hopefully clean commit for this PR
Added check for empty path to avoid SIGSEGV in path parser and resolver
Added requested tests for empty path and non-existing manifest.
However signature for StartHTTPServer had changed.
Now it's hacked as so:

	StartHttpServer(api.API, &Server{Addr: "127.0.0.1:8504", CorsString: ""})

* Parse url before resolve when path and ENS is supplied, example
* swarm/api/http proxy server test for retrieval of subpath through get
* Removed nil entry assignment on subtrie leaf in recursive key retrieval
* Cleaned up path-or-no-path condition in proxy server get handler
* swarm: processed with gofmt refers to lash/go-ethereum@90daa7a
* swarm: Added public access method Parse alias to parse
* swarm: processed with gofmt References nolash/go-ethereum@2ec3fd7
* Rename parse to Parse, removed alias

go get github.com/gordonklaus/ineffassign
ineffassign .

* cmd/bzzd: add missing p2p/discovery flags
* cmd/bzzd: fix two bugs crashing bzzd if bootnodes flag given
* cmd/bzzd: make no swap default, renamed flag bzznoswap->bzzswap
* internal/web3ext: correct methods for bzz IPC module
* cmd/bzzd: ethapi param not mandatory. Warning if no blockchain
* cmd/bzzd: correct default IPC modules in help string
* cmd/utils: fix help description for networkId - add Ropsten
* cmd/bzz, swarm/api, swarm/network: add swarm networkId flag
* cmd/bzzd: change nosync flag to sync and BootTFlag

This implements 1b & 1c of EIP150 by adding a new GasTable which must be
returned from the RuleSet config method. This table is used to determine
the gas prices for the current epoch.

Please note that when the CreateBySuicide gas price is set it is assumed
that we're in the new epoch phase.

In addition this PR will serve as temporary basis while refactorisation
in being done in the EVM64 PR, which will substentially overhaul the gas
price code.

swarm/storage: Allow EOF at the end of the reader in the chunker. Handle the case when Read returns less than length of target slice

* brokenLimitedReader gives error after half size
* TestRandomBrokenData tests chunker with broken reader
* add blocking quitC (instead of errC) and use errC only for errors
* don't close chunkC in tester Split,
* use quitC to quit chunk storage loop

This change imports the Swarm protocol codebase. Compared to the 'swarm'
branch, a few mostly cosmetic changes had to be made:

* The various redundant log message prefixes are gone.
* All files now have LGPLv3 license headers.
* Minor code changes were needed to please go vet and make the tests
  pass on Windows.
* Further changes were required to adapt to the go-ethereum develop
  branch and its new Go APIs.

Some code has not (yet) been brought over:

* swarm/cmd/bzzhash: will reappear as cmd/bzzhash later
* swarm/cmd/bzzup.sh: will be reimplemented in cmd/bzzup
* swarm/cmd/makegenesis: will reappear somehow
* swarm/examples/album: will move to a separate repository
* swarm/examples/filemanager: ditto
* swarm/examples/files: will not be merged
* swarm/test/*: will not be merged
* swarm/services/swear: will reappear as contracts/swear when needed