http: move http_parser functions into a class

CID 1527716: (#1 of 1): Initialization or destruction ordering is
unspecified (GLOBAL_INIT_ORDER)

"destructor_uses_global_object: The destructor of global object
g_context_list itself makes use of global object g_logfp defined in
another compilation unit. The order of destruction is unspecified, so
the destructor of g_context_list might be called after g_logfp has
already been destroyed."

~http_context can call mlog(), but this knows how to deal with
g_logfp==nullptr, so it is a false positive. Nevertheless, move
http_parser functions into a class to contain the globals and wrap
with optional<> so there is only one global.

Author: jengelh