Close postgresql connections when the thread pool thread is idle

I think the idle connections associated with idle threads are still taking up
memory, so especially now that you can configure an arbitrary number of
threads (and thus connections), I think it's good to close them regularly.

guix-data-service/database.scm

@@ -27,6 +27,7 @@
 
             with-postgresql-connection-per-thread
             with-thread-postgresql-connection
+            close-thread-postgresql-connection
 
             with-postgresql-transaction
 
@@ -146,6 +147,15 @@
 
           (f conn)))))
 
+(define (close-thread-postgresql-connection)
+  (let ((conn (fluid-ref %thread-postgresql-connection)))
+    (when conn
+      (pg-conn-finish conn)
+      (hash-remove! (%postgresql-connections-hash-table)
+                    (current-thread))
+      (fluid-set! %thread-postgresql-connection
+                  conn))))
+
 (define* (with-postgresql-transaction conn f
                                       #:key always-rollback?)
   (exec-query conn "BEGIN;")