Persistence

[ruffrey]

• a running Dracula server can optionally take (in)frequent snapshots of Dracula store and write to disk
• Dracula server can load from snapshot on startup
• persistent snapshotting should not affect performance of Dracula store reads/writes (or very minimally affect)
• persistence can lose data and be behind - it is best effort
• snapshot on shutdown signal

[johnpatek]

@ruffrey please assign this issue to me. I am working on a solution that would solve this problem.