prserv/db: Threading fixes

Enabling threading for the PRServer causes a number of issues. Firstly is
the obtuse error:

sqlite3.InterfaceError: Error binding parameter 0 - probably unsupported type

which is due to the class not being derived from object. See:
http://docs.python.org/2/library/sqlite3.html#registering-an-adapter-callable

Secondly, we want to enable multithreadded access to the database so we do this
when we open it. This opens the way up to multithreading the PR server.

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

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