Introduce global timed handlers

There are situations when applications need more predictable timed
handlers that don't depend on connection status. Other usecase of
the global handlers is to manage offline connections, for example,
reconnect after a disconnection.

src/ctx.c

@@ -419,6 +419,7 @@ xmpp_ctx_t *xmpp_ctx_new(const xmpp_mem_t *const mem,
             ctx->log = log;
 
         ctx->connlist = NULL;
+        ctx->timed_handlers = NULL;
         ctx->loop_status = XMPP_LOOP_NOTSTARTED;
         ctx->rand = xmpp_rand_new(ctx);
         ctx->timeout = EVENT_LOOP_DEFAULT_TIMEOUT;