bitbake: prserv: Use WAL mode

bitbake: prserv: Use WAL mode

Ideally, we want the PR service to have minimal influence from
queued disk IO. sqlite tends to be paranoid about data loss and
locks/fsync calls. There is a "WAL mode" which changes the journalling
mechanism and would appear much better suited to our use case.

This patch therefore switches the database to use WAL mode. With this
change, write overhead appears significantly reduced.

(Bitbake rev: 90b05e79764b684b20ce8454e89f05763b02ac97)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>