Move invocation logging into a thread pool executor

[tonybaloney]

This change moves the logging call inside the function invocation callback into a thread pool executor (With a max size of 1).
Effectively allows the logging call to happen concurrently with the function invocation.

In real-terms, with a patched function host, the response times were:

	50	66	75	80	90	95	98	99
Original	15	17	18	20	27	42	58	64
Patched	14	16	17	19	26	35	47	57

At the 95th percentile an improvement of 7ms per request (18%).

IF the customer is using logging inside their function, we should consider making the thread pool executor available/patched on the logger instance.

[tonybaloney]

This should be extended to cover the customer logging function code as well. The Python HTTP function demo spends 70% of its time on line 7

[tonybaloney]

With the threaded logger.