runqueue: Split runqueue to use bitbake-worker

This is a pretty fundamental change to the way bitbake operates. It
splits out the task execution part of runqueue into a completely
separately exec'd process called bitbake-worker.

This means that the separate process has to build its own datastore and
that configuration needs to be passed from the cooker over to the
bitbake worker process.

Known issues:

* Hob is broken with this patch since it writes to the configuration
  and that configuration isn't preserved in bitbake-worker.
* We create a worker for setscene, then a new worker for the main task
  execution. This is wasteful but shouldn't be hard to fix.
* We probably send too much data over to bitbake-worker, need to
  see if we can streamline it.

These are issues which will be followed up in subsequent patches.

This patch sets the groundwork for the removal of the double bitbake
execution for psuedo which will be in a follow on patch.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
runqueue: Move the bitbake-worker execution to a higher level

The worker was being executed by each execution queue so would get
constructed twice for each build. This is wasteful so move execution
to the main runqueue so we only have to start the worker once.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
runqueue: Abstract the start and teardown worker functions

We're going to need a fakeroot/pseudo version of the worker so
abstract the code to start the worker process.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
runqueue: Spawn a separate worker for fakeroot tasks

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
prserv: Adapt autostart to bitbake-worker

With the change to bitbake-worker we need to ensure the workers know
how to contact the PR service, the magic 0 port and singleton is
no longer enough.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake: Bump minimum python version to 2.7.3

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake: Update version to 1.19.1

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