bitbake: cooker/hashserv: Allow autostarting of a local hash server using BB_HASHSERVE

bitbake: cooker/hashserv: Allow autostarting of a local hash server using BB_HASHSERVE

Its useful, particularly in the local developer model of usage, for
bitbake to start and stop a hash equivalence server on local port,
rather than relying on one being started by the user before the build.

The new BB_HASHSERVE variable supports this.

The database handling is moved internally into the hashserv code so that
different threads/processes can be used for the server without errors.

(Bitbake rev: a4fa8f1bd88995ae60e10430316fbed63d478587)

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