BUG/MEDIUM: proxy: ensure pause_proxy() and resume_proxy() own PROXY_LOCK

BUG/MEDIUM: proxy: ensure pause_proxy() and resume_proxy() own PROXY_LOCK

There was a race involving hlua_proxy_* functions
and some proxy management functions.

pause_proxy() and resume_proxy() can be used directly from lua code,
but that could lead to some race as lua code didn't make sure PROXY_LOCK
was owned before calling the proxy functions.

This patch makes sure it won't happen again elsewhere in the code
by locking PROXY_LOCK directly in resume and pause proxy functions
so that it's not the caller's responsibility anymore.
(based on stop_proxy() behavior that was already safe prior to the patch)

This should be backported to stable series.
Note that the API will likely differ < 2.4