PullRequestMergedEvent is no longer asynchronous

Starting from Stash 3.7.0, PullRequestMergedEvent is no longer raised asynchronously. Instead, if it raised directly on the merging thread. This has 2 very important consequences:

1. Any database operations performed by listeners are now performed in the read-only transaction used by the merging thread and are rolled back when the merge call completes
2. Operations performed by event listeners happen in user time and block the merge request from returning until they are complete