Change default max threads if SmartThreadPool is used as the main thread pool from 15 to 300

Running out of such threads under heavy load causes delayed packet processing which can lead to spurious UDP resends and knock on issues.
We already massively boost the min/max builtin pool worker and IOCP threads (which even with STP are still used for inbound network requests) without obvious adverse effects.
The threads are only instantiated if they are required.
This change does not affect other async_call_method options.