job queue: fix loss of finalized opcode result

Currently, unclean master daemon shutdown overwrites all of a job's
opcode status and result with error/None. This is incorrect, since the
any already finished opcode(s) should have their status and result
preserved, and only not-yet-processed opcodes should be marked as
‘error’. Cancelling jobs between opcodes does the same (but this is not
allowed currently by the code, so it's not as important as unclean
shutdown).

This patch adds a new _QueuedJob function that only overwrites the
status and result of finalized opcodes, which is then used in job queue
init and in the cancel job functions. The patch also adds some comments
and a new set constants in constants.py highlighting the finalized vs.
non-finalized opcode statuses.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>