LU-12871 mdd: enable Changelog garbage collection

LU-12871 mdd: enable Changelog garbage collection

Enable the Changelog garbage collection by default.

This feature was disabled by default in commit v2_10_56_0-2-g3442db6fa
(2.11.0) and was fixed in commit v2_11_52_0-59-g31fef6845e (2.12), but
was not re-enabled again by default.

Fixes: 31fef6845e8b ("LU-10680 mdd: create gc thread when no current transaction")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Id4e68e0563cb2216d56bb9aec3a49c83c93ebbe5
Reviewed-on: https://review.whamcloud.com/36467
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>