LU-12098 mdd: explicitly clear changelogs on deregister

LU-12098 mdd: explicitly clear changelogs on deregister

In case of MDS crash in the middle of changelog_deregister, the system
can end up with the changelogs user deregistered, but the changelog
entries not actually cleared. Then the only way to get rid of the
remaining changelogs not used anymore by any user is to register a new
changelogs user and then deregister it.
To protect from this scenario, explicitly clear changelogs used by the
user, before actually deregistering it.

Also add recovery-small test_136 for non-regression purpose.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I14576180c9351337fc4d9ed0e1b176d352584750
Reviewed-on: https://review.whamcloud.com/34688
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>