Expose cluster metadata and token ranges

[cs-sotescu]

This PR does a few things:

• It refactors the cluster metadata handling by extracting it from the TokenAwareHostPolicy into its own manager;
• It manages cluster metadata for any host policy, in the session itself;
• It exposes the metadata through a new Session.ClusterMetadata method;
• It gives access to the token ring thorough ClusterMetadata.GetTokenRing;
• It publishes TokenRing.GetHostForToken and adds a new method, TokenRing.GetTokenRanges.
• The goal is to allow people to build host-targeted token range queries like SELECT id,foo FROM mytable WHERE TOKEN(id)>$startToken AND TOKEN(id)<=$endToken.

[martin-sucha]

Thanks for the pull request and sorry for the late reply! I think this could be useful for full table scans, for example.

[martin-sucha]

How will the API behave in case the token ring is not available (e.g. because of DisableInitialHostLookup option)? The documentation should explicitly mention that.

[cs-sotescu]

I've updated the documentation for ClusterMetadata.TokenRing to clarify in which instances the token ring is not available. The documentation for the TokenRing.Tokens method now has an example function showing how to build token-based queries using the returned token ranges.