Fix: stream intersection fails on snapshot of cleared session

Fix: stream intersection fails on snapshot of cleared session

Observed issue
==============

In the following scenario:
  lttng create --snapshot
  lttng enable-event -u -a
  lttng start
  taskset -c 0 <tracepoint producing app>
  lttng clear
  taskset -c 0 <tracepoint producing app>
  lttng snapshot record
  lttng destroy

When using the stream-intersection mode, babetrace complains that the
time range for the intersection is invalid since the begin timestamp is
after the end timestamp.

This is caused by the presence of "inactive" streams for which no events
are recorded between the clear action and the recording of the snapshot.
These streams have a begin timestamp roughly equal to the moment when
the snapshot was taken (i.e. the end timestamp). Babeltrace, in
stream-intersection mode, attempts to use the latest beginning timestamp
of all streams as the start of the intersection and the earliest end
timestamp as the end boundary.

Path                                                        │File size        │Packets     │Timestamp: beginning         │Timestamp: end               │
snapshot-1-20200622-212617-1/ust/uid/1000/64-bit/channel0_0 │        4.000 KiB│           1│2020-06-22 21:26:01.903685878│2020-06-22 21:26:17.630456312│
snapshot-1-20200622-212617-1/ust/uid/1000/64-bit/channel0_1 │        4.000 KiB│           1│2020-06-22 21:26:17.630909310│2020-06-22 21:26:17.630909310│
snapshot-1-20200622-212617-1/ust/uid/1000/64-bit/channel0_2 │        4.000 KiB│           1│2020-06-22 21:26:17.631295033│2020-06-22 21:26:17.631295033│
snapshot-1-20200622-212617-1/ust/uid/1000/64-bit/channel0_3 │        4.000 KiB│           1│2020-06-22 21:26:17.631673614│2020-06-22 21:26:17.631673614│

Cause
=====

The packet beginning timestamps of the buffers are initialized on
creation (on the first "start" of a tracing session). When a "clear" is
performed on a session, all open packets are closed and the existing
contents are purged.

If a stream is inactive, it is possible for no packet to be "opened"
until a snapshot of the tracing session is recorded.

Solution
========

A new consumer command, "open channel packets" is added. This command
performs a "flush empty" operation on all streams of a channel.

This command is invoked after a clear (after the tracing is re-started)
and on start. This ensures that streams are opened as soon as possible
after a clear, a rotation, or a session start.

Known drawbacks
===============

In the case of an inactive stream, this results an extra empty packet at
the beginning of the inactive streams (typically 4kB) in the snapshots.

In the case of an active stream, this change will cause the first packet
to be empty or contain few events. If the stream is active enough to
wrap-around, that empty packet will simply be overwritten.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I19b5c630fa8bddfb13c3c10f86c6cc9dc4990b08