Attempt to move all database access to main thread.
Single rundb thread working for restarts and reloads.

 * also fixes out of order message setting of db status.
Remove redundant db calls.
cylc/cylc#615: fix hold state persistence after reload
Merge pull request #617 from benfitzpatrick/615.gcylc-hold-reload-fix

#615: fix hold state persistence after reload
Merge pull request #613 from arjclark/db-single-access-hack

cylc: single db access object