Add timestamps to Blocks

[nick-mobilecoin]

Add a ConsensusValue to contain timestamps. This timestamp is
populated when a node initially proposes a set of values. Each node that
proposes values will propose a time. The largest time that is validated
by all the nodes during consensus will be used in the block.

[nick-mobilecoin]

Current dependencies on/for this PR:

• main
  • PR Add timestamps to Blocks #3711 👈

This stack of pull requests is managed by Graphite.

[nick-mobilecoin]

Closing in favor of #3733
This approach doesn't work because all of the ConsensusValues are in a common pool that SCP uses to nominate and ballot values.
If SCP pulls the timestamp value from the pool and nomination or balloting fails, then when SCP tries to propose a second time there will be no timestamp in it's proposal. If this happens for all nodes, then there will be no timestamp to externalize to the block.

SCP treats the ConsensusValues as opaque so it would need to call back to the byzantine ledger worker to get/ensure there is a timestamp in every nomination/ballot.