Prepare sessions for r/w tx can be bottleneck

[olavloite]

Preparing sessions for read/write transactions can be a bottleneck when a burst of write requests need to be executed. This happens when the executor used to prepare sessions for r/w transactions is overloaded with more requests than it can efficiently handle, and causes a queue to build up.

Instead of only relying on the background executor, the session pool should also allow in-process creation of read/write transactions when the background executor is already full.