Increase default number of NonIO threads

Recent performance analysis of SyncWrite-heavy workloads has shown that the default number of NonIO threads can be insufficient to fully utilise the machine - particularly if the CPU core count is relatively low (e.g. 8 cores).

Currently we size the NonIO thread pool based on a CPU coefficient of 0.3 :-

nonio_thread_count = 0.3 * number_of_cpu_cores

For an 8 core machine this gives a value of 2.4 which is rounded to 2.

Note that the current coefficient of 0.3 was set prior to SyncWrites, where NonIO tasks were not needed to perform front-end operations.

Empirical testing has shown that increasing this to 4 NonIO threads on an 8 core system (i.e. coefficient of 0.5) makes better use of the overall machine resources, and doesn't seem to negatively impact other running threads.