As already noted in the design document, an opcode's priority is
increased when the lock(s) can't be acquired within a certain amount of
time, except at the highest priority, where in such a case a blocking
acquire is used.

A unittest is provided. Priorities are not yet used for acquiring the
lock(s)—this will need further changes on mcpu.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>