cylc task-message: unified message queue

[benfitzpatrick]

Each task currently has its own individual message queue, which contains a relatively expensive Queue.Queue object. As we get more and more tasks in a suite, this becomes increasingly unwieldy to keep in memory and to keep checking for new messages.

This change rolls the queues up into a single task pool message queue. It saves around 20% of the per-task memory for the minimal tasks I've tested, which is around 10% of the total memory usage - a surprising amount. The CPU usage is reduced by a couple of percent for a 'super-busy' suite, largely just due to not calling qsize as much.

Due to the communication change, this will break daemon-task mismatched versions before and after this PR, so I've initially put this in the soon milestone.

I've also added a proper profiling behaviour on running with the --profile switch - it is kind of a pain to keep git stashing and stash applying this bit of code!

@hjoliver, @matthewrmshin please review.

[matthewrmshin]

Looks OK 2. Tests OK (with and without global configuration) in my environment.

[hjoliver]

Review 1 - good, tests OK here too.

this will break daemon-task mismatched versions before and after this PR, so I've initially put this in the soon milestone.

This is good to merge as far as I'm concerned, so I'll assign back to @matthewrmshin to make the decision - next release or not - based on your site requirements.

[matthewrmshin]

I'll merge, as this change appears to make things more stable in various tests.