Improve suite state dump logic

State dumps tagged with UTC time at creation.
Symbolic link "state" points to latest.
Roll using a simple FIFO logic.
Dumps from before restart are left untouched.
On restart, create a special symbolic link to point to the used state dump.
Remove some unnecessary method calls.
Merge branch 'master' into improve-state-dump

Conflicts:
	bin/cylc-restart
Merge pull request #825 from matthewrmshin/improve-state-dump

Improve suite state dump logic