lws_metrics

There are a few build options that are trying to keep and report
various statistics

 - DETAILED_LATENCY
 - SERVER_STATUS
 - WITH_STATS

remove all those and establish a generic rplacement, lws_metrics.

lws_metrics makes its stats available via an lws_system ops function
pointer that the user code can set.

Openmetrics export is supported, for, eg, prometheus scraping.