zipkin-hbase and zipkin-web bugfixes

* For reasons as yet unclear, zipkin-web would sometimes inject an empty annotations query string into its location. Passing this empty string to the query service yeilded errors trying to map "" to an annotation id, so we ignore such empty queries.

* The HBase storage mechanism scanned backwards. Since row keys for a particular service start at Long.MAX_VALUE at the epoch and grow towards 0 as time progresses, starting at 0 and scanning until a reversed timestamp actually finds traces after the specified date, and not before.