Update Metadata.get_replicas API

[dkropachev]

Metadata User-facing getReplicas API changes

If driver have user-facing API to get replica list we need to adapt it to tablets:

1. To accomodate tablet requirements new optional parameter table should be added, alternatively another method could be implemented with additional table parameter.
2. If target table is not tablet-based it should return old vnode-based result
3. If target table is tablet-based it should return known tablet replicas for given token
4. if there is no tablet record for given token, it should return empty list