Fix: relayd: harmonize path format in backward-compat mode

Fix: relayd: harmonize path format in backward-compat mode

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

Currently, the relay daemon produces the following path formats
depending on the whether a tracepath is provided, the version of the
session daemon peer, and the grouping option specified on launch of
the relay daemon.

Hostname grouping, no custom trace path
pre 2.11: $BASE/$SHOSTNAME/$SESSION-$DATETIME
    2.12: $BASE/$SHOSTNAME/$SESSION-$DATETIME

Hostname grouping, custom trace path
pre 2.11: $BASE/$HOSTNAME/$TRACEPATH
    2.12: $BASE/$HOSTNAME/$TRACEPATH

Tracing session grouping, no custom trace path
pre 2.11: $BASE/$SESSION/$HOSTNAME-$DATETIME
    2.12: $BASE/$SESSION/$HOSTNAME-$DATETIME

Tracing session grouping, custom trace path
pre 2.11: $BASE/$SESSION/$HOSTNAME/$TRACEPATH
    2.12: $BASE/$SESSION/$HOSTNAME-$DATETIME/$TRACEPATH

As you can see, there is a single case where the format
diverges based on the version of the session daemon.

Cause
=====

Pre-2.11 session daemons do not transmit a session creation time when
a TRACEPATH is specified as part of the streaming url (e.g.
`lttng create my_session --set-url net://localhost/a_path`)

Hence, the backward compatibility path formatting code does not
insert a "DATETIME" string in the resulting path.

Solution
========

The relay daemon samples the time when it creates its session and that
time is formatted into the DATETIME representation if no DATETIME is
present in the path provided by pre-2.11 peers.

Drawbacks
=========

Sampling the relay session creation time will not yield the exact same
behaviour as what a 2.11+ peer would produce, but it is a reasonable
approximation for most use-cases.

Users depending on this time being the exact same as that sampled by
the session daemon will need to adapt tools anyhow if they use the new
--group-output-by-session option, so the change doesn't introduce more
problems.

This behaviour can be surprising when snapshots are streamed by
pre-2.11 peers as the session creation DATETIME will be different for
all snapshots. This is not ideal, but still less jarring than getting
a completely different path format depending on a peer's version.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3316aa35a34985e83ae759851af3a899b0011789
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>