* In order to avoid duplicates of same 'join' tasks we now use
named locks to exclusively create a task execution for 'join'
tasks in DB. Transaction that does that is always separate
from task completion logic and is very short. This is needed
to eliminate DB contention on same records of task execution
table. This is also a reason to use a separate mechanism such
as named locks, and additionally this reduces a number possible
scenarios for getting into deadlocks because for task executions
we have too many different access patterns that can lead to them
in case of doing locking on right on their table records.
So this approach guarantees that there's only one transaction
creates a new task execution object for 'join' task and schedules
'refresh_task_state' job that check 'join' completion.
* Dropped scheduler 'unique_key' column with unique constraint
because in practice it causes DB deadlocks (at least on MySQL)
while simultaneously inserting and updating the table
* Instead of 'unique_key' column we added non-unique 'key' column
that can potentially be used for squashing delayed calls
by scheduler itself (not implemented yet)
* Adjusted Scheduler implementation and tests accordingly
* Fixed task() YAQL function to work without precisely resolve
task execution object in case it's called for the current
task. Previously it was dependent on the luck and we were
lucky enough that tests were passing.
* Increased length of 'unique_key' column for task executions to
250 which is close to a limit for string fields participating
in unique constraints.
Change-Id: Ib7aaa20c2c8834ab0f2d9c90457677c9edb62805
1b0f0cddd6