Fix: sessiond: ust: deadlock with per-pid buffers

Fix: sessiond: ust: deadlock with per-pid buffers

Do not hold the registry lock while communicating with the consumerd,
because doing so causes inter-process deadlocks between consumerd and
sessiond with the metadata request notification.

The deadlock involves both sessiond and consumerd:

* lttng-sessiond:

thread 11 - thread_application_management

close_metadata()
  pthread_mutex_lock(&registry->lock);
  consumer_close_metadata()
    pthread_mutex_lock(socket->lock);

thread 15 - thread_consumer_management

ust_consumer_metadata_request()
  pthread_mutex_lock(&ust_reg->lock);

thread 8 - thread_manage_clients

consumer_is_data_pending
  pthread_mutex_lock(socket->lock);
  consumer_socket_recv()

* lttng-consumerd:

thread 4 - consumer_timer_thread

sample_channel_positions()
  pthread_mutex_lock(&stream->lock);

thread 8 - consumer_thread_sessiond_poll
  consumer_data_pending
  pthread_mutex_lock(&consumer_data.lock);
  pthread_mutex_lock(&stream->lock);

thread 7 - consumer_thread_data_poll

lttng_consumer_read_subbuffer
  pthread_mutex_lock(&stream->chan->lock);
  pthread_mutex_lock(&stream->lock);
  do_sync_metadata
    pthread_mutex_lock(&metadata->lock);
    lttng_ustconsumer_sync_metadata
      pthread_mutex_unlock(&metadata_stream->lock);
      lttng_ustconsumer_request_metadata()
        pthread_mutex_lock(&ctx->metadata_socket_lock);
        lttcomm_recv_unix_sock()

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>