bitbake: Always use separate process for PR Service

Using the threading module interacts badly with multiprocessing used elsewhere
in bitbake under certain machine loads. This was leading to bitbake hanging on
Ctrl+C when the PR Server was being used.

This patch converts it to always use the daemonize code which
then means the threading code isn't required.

[YOCTO #3742]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
prserv/serv.py: Fix logging in daemon mode

In deamon mode we need to ensure the logging module is sending log data to the
log file. These changes ensure this happens correctly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>