bitbake: cooker: Handle parse threads disappearing to avoid hangs

If one of the parse threads disappears during parsing for some reason, bitbake
currently hangs. Avoid this (and zombie threads hanging around) by joining()
threads which have exited.

(Bitbake rev: 920111a330be59e5be2068a8f1a9edcbc6c14402)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit dc86a533d951d13643ce446533370da804782afc)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>