sqlite: implement batched async operation scheduling

Trade a bit of db operation latency for throughput by batching db
operations together before scheduling their execution on the thead pool.
Completion notifications are currently delivered eagerly through an
additional `uv_async` handle, but this is not strictly necessary for the
chosen implementation strategy and might get removed again.

Some care has been taken to not criss-cross memory allocations and de-
allocations between threads as modern modern allocators tend to perform
worse in such scenarios.

Author: BurningEnlightenment