whisper: project restructured, version 5 introduced
This commits adds a draft version of the new shh v5 protocol.
The new version is not on by default, --shh still selects version 2.
These tests have become a common annoyance on CI. Fix them by allowing
messages with expiration == now into the cache and delaying the check
for expired message handling slightly.
Whisper's expire and broadcast loops happen in two separate go routines.
Whenever an envelope is being expired it's removed from the set of
envelopes and it looses all information about the envelope, including
the "known hash". After the envelope has been removed it can be
re-accepted by a broadcasting peer putting back the envelope in the set
of envelopes. Since the envelope broadcast loop is separate of the
expire loop expired messages may be broadcast to other peer, resulting
in messages **never** being dropped.
This PR includes an expire check before adding new messages to the set
of envelopes.