x/build: prevent submission of CL with an outstanding wait-* hashtag

[dmitshur]

There is a convention of using a wait-* hashtag to indicate a CL is waiting on some specific item before progress can be made. This is described in issue #10658, which may need to be resolved more before progress can be made here.

I suspect it may be helpful to to have an automatic mechanism that prevents a CL from being submitted if it has an active wait-* hashtag. If waiting is no longer necessary, the wait-* hashtag should be removed. This can help avoid accidental early submission. See CL 260700 for a recent example of a CL that was accidentally submitted too early despite having a wait-release hashtag.

Perhaps it can be implemented as gopherbot applying a -2 code review vote (and a comment?), as a Gerrit rule (similar to implementation of Trust votes), or some other way. This is related to #41722, which is about detecting a mismatch in target branch, #10658, which is about detecting large binaries, and to #18548, which is about various server-side checks.

/cc @golang/osp-team