bitbake: prserv/serv: Multithread the server

bitbake: prserv/serv: Multithread the server

This makes the PR server multithreaded and able to handle multiple connections
at once which means its no longer a build bottle neck when serving one connection
at a time. I've experimented and database connection for each thread seems to
cause the least issues, pushing the contention for sqllite to handle itself.

This means moving the db/table connection code into the actual function methods.
It doesn't abstract well as a function since we need the db object around for
the lifetime of the function as well as the table else we lose the connection.

(Bitbake rev: bf9be2029b2bded5f532bdda4c38ae3dff5d1cf6)

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