user: Track simple statistics about users and server activity.
user: Track simple statistics about users and server activity.

- Uses the expvar package.

All tests pass.
go vet is clean.
No new lint errors.

Change-Id: Ia1b7418496778d6bbd65e6289f549a0268cd645d
Reviewed-on: https://upspin-review.git.corp.google.com/1590
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin