exp/cmd/camserver: implement DirServer.Watch
exp/cmd/camserver: implement DirServer.Watch

Change-Id: I7d523fe6568b413cf273b02999df625c5ce17cc8
Reviewed-on: https://upspin-review.googlesource.com/9824
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/exp,upspin/exp