treat removal of non existing node as successful
jlock undo
set all children complete when parent fails
Check if sqlite really deleted a row using sqlite3_changes(). bb#814

Update volume size using only one query which doesn't require locking
h->db
log a message and don't run GC if we detect a serious problem
bb #386

Don't hardcode settings can be locally tweaked for performance.
Values that should be synchronized cluster-wide or
that are used with STRIFY in queries are still hardcoded (bb #335).
wrap gc_periodic in small transcations

avoid warnings about database locked