mcpu.Processor: Raise exception on lock acquire timeout

Right now the timeout is not passed by any caller, making the code
effectively go back to blocking acquires. Since the timeout is always
None, no caller needs to be changed in this patch.

This change also means that any LUXI query handled by ganeti-masterd
will use blocking acquires if they need locks (only the case for getting
tags).

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