Commits on Jun 13, 2019

1. Lock-lesser transitions without additional queries …

   #350
   
   @arthurnn opened #350 to reduce the impact of Statesman taking gap locks
   when using MySQL. The crux of the issue is that MySQL's implementation
   of REPEATABLE READ can take wide locks when an update touches no rows,
   which happens frequently on the first transition of Statesman.
   
   By first creating the new transition, we can avoid issuing an update
   that will take the large gap lock. This order of queries meant we added
   an additional query to the transition step which could impact people who
   rely on low-latency Statesman transitions.
   
   This commit is another take on the same approach that collapses two
   queries into one, by taking the update of the old and new transition's
   most_recent column and updating them together. It's slightly janky but
   if robust, would be a good alternative to avoid additional latency.

   

   lawrencejones committed Jun 13, 2019
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