database: Use "write-ahead log" mode and set a long "busy timeout".

This should avoid "database is locked" errors when there's a lot of
concurrency, for instance when offloading simultaneously a lot of
builds.

* guix/store/database.scm (call-with-database): Add two 'sqlite-exec'
calls to set 'journal_mode' and 'busy_timeout'.

guix/store/database.scm

@@ -79,6 +79,15 @@ (define (call-with-database file proc)
 create it and initialize it as a new database."
   (let ((new? (not (file-exists? file)))
         (db   (sqlite-open file)))
+    ;; Turn DB in "write-ahead log" mode, which should avoid SQLITE_LOCKED
+    ;; errors when we have several readers: <https://www.sqlite.org/wal.html>.
+    (sqlite-exec db "PRAGMA journal_mode=WAL;")
+
+    ;; Install a busy handler such that, when the database is locked, sqlite
+    ;; retries until 30 seconds have passed, at which point it gives up and
+    ;; throws SQLITE_BUSY.
+    (sqlite-exec db "PRAGMA busy_timeout = 30000;")
+
     (dynamic-wind noop
                   (lambda ()
                     (when new?