mcpu: Implement lock timeouts

The timeout is always between ~0.1 and ~10.0 seconds. A small
variation of ±5% is added to prevent different jobs from
fighting each other. After 10 attempts to acquire the locks with
a timeout, a blocking acquire is made.

Lock status reporting will be improved in a separate patch.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>