CUSTOM: relayd protocol: ignore reply on relayd_send_index and relayd_send_close_stream

CUSTOM: relayd protocol: ignore reply on relayd_send_index and relayd_send_close_stream

Note: this patch is not a bugfix, it is a targeted modification to
improve timing and predictability for particular scenarios.

Note: this patch only applies to userspace tracing.

Part of this work might find itself upstream but most probably not in
this form. Deeper work should be done upstream to mitigate the real root
of the problem which is how the network protocol for live works and
lifetime management of the different component required for the live
feature.

Scenario:
=======
   System with high CPU resource restriction (base high workload 99% cpu load),
   High CPU count (32),
   Slowish network (ping ~60-100ms),
   Timing constraint for create, configure, start, stop , destroy , create ... cycles.

A lot of time is wasted waiting for response that do not provide more
information and are essentially not pertinent. Here the focus is done on
the reply for relayd_send_index and relayd_send_close_stream.

relayd_send_index is used at the end of a buffer consumption and do not
require further protocol synchronization on lttng-sessiond side. This
allows us to simply mark the bytes of the response as `to ignore` on the
next real receive operation.

relayd_send_index is also used to send empty index during the live
timer. Again there is now direct value in waiting for the reply here
since tcp guarantee send/receive ordering.

relayd_send_close_stream again does not benefit from waiting on the
reply considering that even in the possibility of error there is not
much we can do since we are closing it on lttng-sessiond side.

NOTE:
   Three call sites are responsible for "purging" the unwanted reply:
   recv_reply, recv_reply_ignore and relayd_close.

   recv_reply simply try to receive all bytes to ignore at the time it
   is called. This is to preserve the protocol integrity.

   recv_reply_ignore will issue a recv call if a cutoff for `bytes to
   ignore` (128 currently) is met. The recv call will recv and discard
   the hald the quantity of the cutoff (64 in this patch). This ensure
   that long period with no real receive operation do not lead to buffer
   bloat. We slowly consume data that is "useless". The amount we
   receive is totally arbitrary and is only set to half of the cutoff to
   `give a chance to the runner` since there is a high probability that
   they are available. In any case, we block if it is not the case.

   relayd_close must wait for all `ignored bytes` to ensure that we do
   not close the socket before the relayd have sent all its reply.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I5da9b364c73498e8c0156c71960432229932ecb5