Check that probes have satisfactory timing levels

[mattcolegate]

Issue #590 was caused by the http(s) probes using the timing contexts at the functional level (stored in aspect.js). This is fine for probes that can expect big enough gaps in their event flow that the events pass singularly through the instrumented function, but not for probes whose events are rapid. The probes should be examined for speed, and if necessary have their own timing contexts stored at the individual request level to prevent clashes of timing data. This may require the wholesale removal of the context object from aspect.js, which would require thorough testing as it would constitute a change to the underlying framework of Appmetrics.