Adds a message filter.
Filters are handlers that are run on all incoming and outgoing messages, prior to standard dispatch. Filters are run in the order that they were added. The same handler can be added as a filter more than once, in which case it will be run more than once. Filters added during a filter callback won't be run on the message being processed. Filter functions are allowed to modify and even drop messages.
Note that filters are run in a dedicated message handling thread so they can't block and, generally, can't do anything but signal a worker thread. Also note that filters are rarely needed - use API such as send_message_with_reply, signal_subscribe or call instead.
If a filter consumes an incoming message the message is not dispatched anywhere else - not even the standard dispatch machinery (that API such as signal_subscribe and send_message_with_reply relies on) will see the message. Similarly, if a filter consumes an outgoing message, the message will not be sent to the other peer.
user_data_free_func is non-null, it will be called (in the thread-default main context of the
thread you are calling this method from) at some point after
user_data is no longer needed. (It is not guaranteed to be called
synchronously when the filter is removed, and may be called after this has been destroyed.)
a filter function
user data to pass to
function to free
a filter identifier that can be used with remove_filter