Wire up buffer load routine from persister and wal

[pauldix]

On server startup to load up the buffer we will need to:

1. Have the persister load the Catalog
2. Have the persister load the most recent segment files
3. Have the buffer load up the WAL for any segment id > the last persisted segment file
4. If there are more than 1 wal segments to load, persist older segments before continuing
5. Now we're ready to serve