Use WAL journal mode for sqlite

Use PRAGMA journal=WAL (keeping synchronous=NORMAL as is in db.py already) and remove the in-house pending change log for sqlite container listing updates. The sqlite3 wal file serves a similar function except (a) read queries can use it on the fly without having to update the core database/indexes, avoiding fsync() and contention; and (b) when the wal log is merged in the "checkpoints" they still don't block readers (e.g. container listings). The might require other PRAGMA statements to avoid affecting durability of other table changes aside from the 'object' one. Also the rsync code to fix corrupt DBs would need adjustment.