key/server: read log from storage before appending to it
key/server: read log from storage before appending to it

This bug created some inconsistencies in our key server log as the
incremental hash was reset to zero when the server restarted.
We need to decide how to remediate this, but until then this at least
fixes all log entries from now on.

Change-Id: I4f3f8b90eacd2498b5b247d35978b8756c2da3e8
Reviewed-on: https://upspin-review.googlesource.com/8283
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin