* 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

* whisper: fix empty topic

* whisper: add check to matchSingleTopic

* whisper: add tests

* whisper: fix gosimple

* whisper: added lastTopicByte const

This is the main feature of v6.

Fixes #15777 because null is now allowed for hexutil.Bytes.

New Whisper-level message introduced (PoW requirement),
corresponding logic added, plus some tests.

Changed the communication protocol for ordinary message,
according to EIP 627. Messages will be send in bundles, i.e.
array of messages will be sent instead of single message.

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.

* build: enable unconvert linter

 - fixes #15453
 - update code base for failing cases

* cmd/puppeth: replace syscall.Stdin with os.Stdin.Fd() for unconvert linter