Make mg_connect_client() thread-safe
Make mg_connect_client() thread-safe

common_client_context has been used not "Only for memory statistics",
then it affects thread-safety.
Each client now has own mg_context (common_client_context was
introduced before that), so it seems already unnecessary.

GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver