Make it remember when timeline starts in general and on this safekeeper in
particular (the point might be later on new safekeeper replacing failed one).
Bumps control file and walproposer protocol versions.
While protocol is bumped, also add safekeeper node id to
AcceptorProposerGreeting.
ref #1561
Commit edba2e97 renamed pageserver/README to pageserver/README.md, but
forgot to update links to it. Fix.
Rename libs/postgres_ffi/README and safekeeper/README files to also
have the the .md extension, so that github can render them nicely.
Quote ascii-diagram in safekeeper/README.md so that it renders
correctly.
Remove when it is consumed by all of 1) pageserver (remote_consistent_lsn) 2)
safekeeper peers 3) s3 WAL offloading.
In test s3 offloading for now is mocked by directly bumping s3_wal_lsn.
ref #1403