Make DefaultTabletMap initialize tabletMap later

[dkropachev]

Currently tabletMap is initialized once at DefaultMetadata.EMPTY As result all the session instances have same tabletMap, which can lead to all sort of problems.
This commit postpone tabletMap initialization and changes Metadata API to reflect these changes.

Partially addresses: #504

[Bouncheck]

While this probably works would it not be better to eventually decouple TabletMap from TokenMap and have it work fully separately? Binding tablet map initialization to token map will require in the future that token map is always running too.

Alternatively this can be fixed with just changing

  public static DefaultMetadata EMPTY =
      new DefaultMetadata(
          Collections.emptyMap(), Collections.emptyMap(), null, null, DefaultTabletMap.emptyMap());

into

  public static DefaultMetadata empty() {
      return new DefaultMetadata(
          Collections.emptyMap(), Collections.emptyMap(), null, null, DefaultTabletMap.emptyMap());
}

[Bouncheck]

Ah, its not part of rebuildTokenMap but withNodes. Then i guess its fine.

[dkropachev]

While this probably works would it not be better to eventually decouple TabletMap from TokenMap and have it work fully separately? Binding tablet map initialization to token map will require in the future that token map is always running too.

Alternatively this can be fixed with just changing

  public static DefaultMetadata EMPTY =
      new DefaultMetadata(
          Collections.emptyMap(), Collections.emptyMap(), null, null, DefaultTabletMap.emptyMap());

into

  public static DefaultMetadata empty() {
      return new DefaultMetadata(
          Collections.emptyMap(), Collections.emptyMap(), null, null, DefaultTabletMap.emptyMap());
}

They are currently couple via Partitioner, which will be addressed at #502

[dkropachev]

All test failures not related to the PR.