Changes in #15811 broke partial topic filtering. Re-enable it.

* keystore, ethash, eth, miner, rpc, whisperv6: tech debt with now defunct set.

* whisperv5: swap out gopkg.in/fatih/set.v0 with supported set

 #15685 made peer_test.go more portable by using random free port instead of hardcoded port 30303 (#15687)

Improves test portability by resolving 127.0.0.1:0
to get a random free port instead of the hard coded one. Now
the test works if you have a running node on the same
interface already.

Fixes #15685

ToECDSAPub was unsafe because it returned a non-nil key with nil X, Y in
case of invalid input. This change replaces ToECDSAPub with
UnmarshalPubkey across the codebase.

Fixes #16756

* whisper/mailserver: pass init error to the caller

* whisper/mailserver: add returns to fmt.Errorf

* whisper/mailserver: check err in mailserver init test

- Fixes #16271. What was appeneded was a pointer to
an object that changes during the iteration.
- The topic is allocated as a 4-byte array, fill partial topics
with 0s. Partial topics are currently disabled, but would
crash as they rely on the presence of byte number 3.

The parameter `rand` is unused in `PrivateKey.Decrypt`. Decryption in
the ECIES encryption scheme is deterministic, so randomness isn't
needed.

whisper: final refactoring

* whisper: timeout extended

* whisper: test updated

* whisper: test updated

The bulk of the issue was to adapt to the new requirement
that a v6 filter has to either contain a symmertric key or
an asymmetric one.

This commits revert one of the fixes that I made to remove
a linter warning: unexporting NewSentMessage. This is not
really a problem as I have a cleanup in the pipe that will
solve this issue.

* whisper: bloom filter refactoring

* whisper: fixed full node

* whisper: message format changed

* whisper: tests fixed

* whisper: style fixes

* whisper: fixed names, fixed failing tests

* whisper: fix merge issue in #15870

Occured while using the github online merge tool. Lesson learned.

* whisper: fix a gofmt error for #15870

* whisper: fixes warnings from the code linter

* whisper: more non-API-breaking changes

The remaining lint errors are because of auto-generated
files and one is because an exported function has a non-
exported return type. Changing this would break the API,
and will be part of another commit for easier reversal.

* whisper: un-export NewSentMessage to please the linter

This is an API change, which is why it's in its own commit.
This change was initiated after the linter complained that
the returned type wasn't exported. I chose to un-export
the function instead of exporting the type, because that
type is an implementation detail that I would like to
change in the near future to make the code more
readable and with an increased coverage.

* whisper: update gencodec output after upgrading it to new lint standards