swarm/fuse: simplify externalUnmount, use subtests

This commit does various code refactorings:

- generalizes and moves the request retrieval/timeout/resend logic out of LesOdr
  (will be used by a subsequent PR)
- reworks the peer management logic so that all services can register with
  peerSet to get notified about added/dropped peers (also gets rid of the ugly
  getAllPeers callback in requestDistributor)
- moves peerSet, LesOdr, requestDistributor and retrieveManager initialization
  out of ProtocolManager because I believe they do not really belong there and the
  whole init process was ugly and ad-hoc

The code looked for /usr/bin/diskutil on darwin, but it's actually
located in /usr/sbin. Fix that by not specifying the absolute path.
Also remove weird timeout construction and extra whitespace.

As per #14661 TransactionByHash always returns false for pending.
This uses blockNumber rather than blockHash to ensure that it returns
the correct value for pending and will not suffer side-effects if
eth_getTransactionByHash is fixed in future.

* ethdb: remove Set

Set deadlocks immediately and isn't part of the Database interface.

* trie: add Err to Iterator

This is useful for testing because the underlying NodeIterator doesn't
need to be kept in a separate variable just to get the error.

* trie: add LeafKey to iterator, panic when not at leaf

LeafKey is useful for callers that can't interpret Path.

* trie: retry failed seek/peek in iterator Next

Instead of failing iteration irrecoverably, make it so Next retries the
pending seek or peek every time.

Smaller changes in this commit make this easier to test:

* The iterator previously returned from Next on encountering a hash
  node. This caused it to visit the same path twice.
* Path returned nibbles with terminator symbol for valueNode attached
  to fullNode, but removed it for valueNode attached to shortNode. Now
  the terminator is always present. This makes Path unique to each node
  and simplifies Leaf.

* trie: add Path to MissingNodeError

The light client trie iterator needs to know the path of the node that's
missing so it can retrieve a proof for it. NodeIterator.Path is not
sufficient because it is updated when the node is resolved and actually
visited by the iterator.

Also remove unused fields. They were added a long time ago before we
knew which fields would be needed for the light client.

cmd/geth: fixed a minor typo in the comments

core/vm: improve opByte

mobile: use EIP155 signer for determining sender

consensus/ethash, cmd/geth: Fix `makedag` epoch

`geth makedag <blocknumber> <path>` was creating DAGs for
`<blocknumber>/<epoch_length> + 1`, hence
it was impossible to create an epoch 0 DAG.

This fixes the calculations in `consensus/ethash/ethash.go` for
`MakeDataset` and `MakeCache`, and applies `gofmt`.

Fixes #14586

The error returned by client.Upload was previously being ignored due to becoming
out of scope outside the if statement. This has been fixed by instead defining a
function which returns the hash and error (rather than trying to set the hash in
each branch of the if statement).

core: Fix VM error logging

Signed-off-by: Lewis Marshall <lewis@lmars.net>

core/vm: Use a bitmap instead of a map for jumpdest analysis

t push --force

This PR adds a Swarm development environment which can be run in a
Docker container and provides scripts for building binaries and running
Swarm clusters.

accounts/keystore, crypto: don't enforce key checks on existing keyfiles

core: reduce transaction reorganization overhead