libwebsockets

ss: take care to free any metadata heap values before overwrite

Break out the core ss_set_metadata action into a subfunction that
takes the lws_ss_metadata_t, and is fixed to retire heap-based
values before they go out of scope, and adapt the exported version
to call through to that.

Simplify extract_metadata() to reuse the subfunction as well, in
both well-known and custom header cases.