Fix: ust metadata: resample clock on regenerate metadata

Fix: ust metadata: resample clock on regenerate metadata

Observed issue
==============
The system test jobs complain that the clock regeneration is broken
since the move to the tsdl visitor approach for metadata generation.

 # Test UST local with metadata regeneration
 # destructive//../../src/bin/lttng/lttng create regen -o /tmp/tmp.metadata_regen_after_data_change.Vzb0vL
 ok 23 - Create session regen in -o /tmp/tmp.metadata_regen_after_data_change.Vzb0vL
 # destructive//../../src/bin/lttng/lttng enable-event tp:tptest -s regen -u
 ok 24 - Enable ust event tp:tptest for session regen
 # destructive//../../src/bin/lttng/lttng start regen
 ok 25 - Start tracing for session regen
 # destructive//../../src/bin/lttng/lttng stop regen
 ok 26 - Stop lttng tracing for session regen
 ok 27 - Validate trace at date 1970-02-02
 # destructive//../../src/bin/lttng/lttng start regen
 ok 28 - Start tracing for session regen
 # destructive//../../src/bin/lttng/lttng regenerate metadata -s regen
 ok 29 - Metadata regenerate regen
 # destructive//../../src/bin/lttng/lttng stop regen
 ok 30 - Stop lttng tracing for session regen
 # destructive//../../src/bin/lttng/lttng destroy regen
 ok 31 - Destroy session regen
 not ok 32 - The trace is not at the expected date
 #   Failed test 'The trace is not at the expected date'
 #   in destructive//../utils/tap/tap.sh:fail() at line 159.

Cause
=====

Previously the clock was sampled on each call to `ust_metadata_session_statedump`
, currently the clock is only sampled on creation of the
`lttng::sessiond::ust::registry_session::registry_session` object.

Solution
========

On `lsu::registry_session::regenerate_metadata`, sample the clock and
replace the registry_session _clock object.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I6f671e4c01f71e1574824236cef03915a9c79b36