classes/buildhistory: reduce parsing time

classes/buildhistory: reduce parsing time

Disable several python functions if not parsing within the worker
context. This avoids executing expensive operations while parsing
recipes (which is unnecessary).

(Thanks to Richard Purdie for pointing out the issue and suggesting the
workaround.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>