Silence asynchronous tags file updates by default

This change is related to pull request #82 however that pull request
wasn't merged here (and won't be merged at all) because it was based on
the old/dead `async-cleanup' feature branch (see pull request #49 on
GitHub) instead of the new `async-take-two' feature branch (see pull
request #84 on GitHub). This change set implements the equivalent on the
new feature branch.

In addition to Ingo's comments in pull request #82, the asynchronous
message frequently disturbs me while typing a Vim command, which is kind
of annoying. If everything goes well and we can get the async mode to be
stable enough to become the default mode then the status messages will
only be interesting when debugging a problem anyway.