Add term history to safekeepers.

Persist full history of term switches on safekeepers instead of storing only the
single term of the highest entry (called epoch). This allows easily and
correctly find the divergence point of two logs and truncate the obsolete part
before overwriting it with entries of the newer proposer(s).

Full history of the proposer is transferred in separate message before proposer
starts streaming; it is immediately persisted by safekeeper, though he might not
yet have entries for some older terms there. That's because we can't atomically
append to WAL and update the control file anyway, so locally available WAL must
be taken into account when looking at the history.

We should sometimes purge term history entries beyond truncate_lsn; this is not
done here.

Per https://github.com/zenithdb/rfcs/pull/12

Closes #296.

Bumps vendor/postgres.

test_runner/batch_others/test_wal_acceptor.py

@@ -392,6 +392,7 @@ def test_sync_safekeepers(zenith_env_builder: ZenithEnvBuilder, pg_bin: PgBin):
                 "lm_prefix": "prefix",
                 "lm_message": "message",
                 "set_commit_lsn": True,
+                "send_proposer_elected": True,
                 "term": 2,
                 "begin_lsn": begin_lsn,
                 "epoch_start_lsn": epoch_start_lsn,