Ensured isolation between the single writer SQLite transactions

[oskardudycz]

Because sqlite is async and missing MVCC in SQLite (different model, message loop), then we can't rely on commands being run sequentially for the shared connection. We do that for the SQLite3 wrapper to improve write performance. This caused several issues:

• command can accidentally become part of other open transactions (e.g. parallel event appends, or append while the consumer is making checkpointing),
• deadlocks for nested connections or transactions being opened and scheduled into the internal task processor of the singleton pool.

Besides that, there may be edge cases where multiple nested transactions were opened in the same nested connection, then they weren't correctly disposed of.

Fixed those issues by ensuring that reentrance of the critical section (so acccesing transaction) will be detected for the same section and fixing the nested transaction disposal.