Stop using a pool of threads for database operations

Now that squee cooperates with suspendable ports, this is unnecessary. Use a
connection pool to still support running queries in parallel using multiple
connections.

guix-data-service/database.scm

@@ -28,6 +28,7 @@
             run-sqitch
 
             with-postgresql-connection
+            open-postgresql-connection
 
             with-postgresql-connection-per-thread
             with-thread-postgresql-connection