consensus: wrap up the spec for PBTS #2019
Description
Originally tendermint/tendermint#8953.
The Proposer-Base Timestamps (PBTS) specification should be updated to:
- Remove versioning from the PBTS spec files. tendermint/tendermint#8602
- address @angbrav comments (PBTS spec feedback (Manuel) tendermint/tendermint#8628)
- address Only wait if a new timestamp is being generated #1206
This issue extends the migration of this spec from #1973