Don't hold walproposer WAL in memory (#141)

WAL is no longer in memory to prevent OOM in the compute. Removed in-memory queue because it's not needed anymore. When streaming, WAL is now read directly from disk. Every safekeeper has a separate XLogReader. walproposer will now read as much WAL as it can for a single AppendRequest message, it can help with recovering lagging safekeepers. Because Recovery needs to save WAL for streaming, now walproposer can write WAL to disk and `--sync-safekeepers` mode will create pg_wal directory if needed. Replication slot `restart_lsn` is now synced with `truncate_lsn` to prevent truncation of disk WAL until needed.

Author: petuhovskiy