4.x: Reimplement DefaultTabletMap initialization #504

Current problem is that all session have shared tabletmap, even if they are targeting different clusters.
Another problem is that on shema refresh, tabletmap does not delete old tablets.
To address that we need to copy of what is currently done for tokenMap:

1. Initialize Metadata with null in tabletMap
2. On DefaultMetadata.withSchema call rebuildTabletMap that would instantiate new fresh tabletMap.
3. Make rebuildTabletMap to cleanup any tablets that have keyspace or tablet that does not exist anymore.