Add support for explicit Sync

[kevburnsjr]

This PR adds one new public method buntdb.(*DB).Sync

WAL Compaction

I'd like to use BuntDB with SyncPolicy: Never as an on-disk state machine behind a Raft proposal log. The raft log acts as a WAL and the raft library calls Sync explicitly on the state machine during WAL compaction.

Without an explicit Sync operation in BuntDB, it's not possible to guarantee write durability. If the WAL is truncated and some updates have not yet been flushed to disk, a crash could cause unsynced writes to be discarded, causing the state machine to become unrecoverable.

Similar To

• lmdb.(*Env).Sync
• pebble.(*DB).Flush
• bbolt.(*DB).Sync

[mitar]

Please add this.