bugfix omelasticsearch: failed.http stats counter had incorrect count
omelasticsearch: add failed.httprequests stats counter
bugfix: omelasticsearch: did not correctly initialize stats counters

This usually did not lead to any problems, because they are in static
memory, which is initialized to zero by the OS when the plugin is
loaded. But it may cause problems especially on systems that do not
support atomic instructions - in this case the associated mutexes also
did not get properly initialized.
Merge branch 'v7-stable'

Conflicts:
	plugins/omelasticsearch/omelasticsearch.c