Provides a DST, and associated metadata as beneficial.
Also utilizes MuSig's context to session-bind. Since set_keys_messages also
binds to set, this is semi-redundant, yet that's appreciated.
SubstrateBlock's provision of the most recently acknowledged block has
equivalent information with the same latency. Accordingly, there's no need for
it.
The signing set should be the first group to submit preprocesses to Tributary.
Re-attempts shouldn't be once every 30s, yet n blocks since the last relevant
message.
Removes the use of an async task/channel in the signer (and Substrate signer).
Also removes the need to be able to get the time from a coin's block, which was
a fragile system marked with a TODO already.
