dir/dircache: log a message whenever successfully starting a watch
dir/dircache: log a message whenever successfully starting a watch

We now log whenever a Watch succeeds. This makes
debugging connectivity problems a bit easier.

Change-Id: I43c1e455dee88b2d2ef80a0d77dcf3ccb5b1db71
Reviewed-on: https://upspin-review.googlesource.com/18520
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin