Remove possible blockages (#74)

This aims at reducing the possibilities of stalling w/ the following:

- Don't interrupt long-running SQLite executions. They're pretty much always necessary. The only way that would be problematic is if we have a deadlock in Corrosion within a transaction.
- Rewrite the compaction logic to do a single-ish query to get all cleared versions
- Keep synchronizing in a loop until a synchronization completes successfully.

Author: jeromegn