sql: use semaphore to limit access to the connection pool

This ensures that if multiple connections are returned
to the pool at the same time, waiters get them in the order
they were placed in the queue.

CHANGELOG.md

@@ -12,6 +12,7 @@
 - deltachat-rpc-server: do not block stdin while processing the request. #4041
   deltachat-rpc-server now reads the next request as soon as previous request handler is spawned.
 - enable `auto_vacuum` on all SQL connections #2955
+- use semaphore for connection pool #4061
 
 ### API-Changes