LU-14663 mdc: start changelog thread upon first access

LU-14663 mdc: start changelog thread upon first access

thus leaving the caller a chance to set CHANGELOG_FLAG_FOLLOW,
otherwise the thread (started from open()) can reach the end
of the changelog and exit early.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ic14b6c991010bbe5197b5a8b0fedf0f4007e98c1
Reviewed-on: https://review.whamcloud.com/43513
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>