fix: fix some race conditions running connection worker

Previously, it was quite possible to have two connection workers
running, or to get into a state where a failed connection worker
is still considered running preventing new connection workers
from starting.

CHANGELOG.md

@@ -41,6 +41,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
    + Can generate the plan for different media types using the `for` parameter: `Accept: application/vnd.pgrst.plan; for="application/vnd.pgrst.object"`
    + Different options for the plan can be used with the `options` parameter: `Accept: application/vnd.pgrst.plan; options=analyze|verbose|settings|buffers|wal`
    + The plan can be obtained in text or json by using different media type suffixes: `Accept: application/vnd.pgrst.plan+text` and `Accept: application/vnd.pgrst.plan+json`.
+   + Limited to generating the plan of a json representation(`application/json`) but can be extended later to allow other representations.
+   + The plan can be obtained in text(`Accept: application/vnd.pgrst.plan+text`) and json(`Accept: application/vnd.pgrst.plan+json` or `Accept: application/vnd.pgrst.plan`) format.
+ - #2397, Fix race conditions managing database connection helper - @robx
 
 ### Fixed