Set modified date at execution

So that ActionScheduler_QueueCleaner::mark_failures() can correctly
set actions running for more than 30 mins as failed (rather than
those which were claimed more than 30 mins ago).
Restrict concurrent batches

Fixes #17
Set cleanup batch size to same size of runner

To avoid taking 10 minutes to clean up a queue of 100 actions.
Prevent duplicate processing of actions

By checking the ActionClaim is still valid for the action before
processing it.
Prevent duplicate processing of actions

By checking the ActionClaim is still valid for the action before
processing it.
Set memory limit to max available
Reduce batch size

Now that failures must be cleaned up and handled by the plugin scheduling
the action, we should try to avoid fatal errors like memory exhaustion or
timeout. Reducing the batch size slows the speed of processing but greatly
reduced the risk of these errors.
Reduce the default timeout period to 5 minutes

Now that we only allow 5 concurrent queues, we don't want to leave
queues clogged up for 30 minutes.
Set all cleanup batch sizes to same as batch size
convert claims to posts instead of strings

actions are claimed by setting the post parent to the claim ID
test for batch limit, and optimize query for claim count