Support default consistency level

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Many C* clients support a default consistency level to use unless overridden. We can support a default consistency level that we pass to Xandra.start_link/1 and fall back to that unless the :consistency_level option is passed explicitly.

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Drivers that do this:

• Java driver: https://stackoverflow.com/a/39839221
• cqlsh: https://docs.datastax.com/en/archived/cql/3.1/cql/cql_reference/consistency_r.html
• erlcass: https://github.com/silviucpp/erlcass (search for default_consistency_level in the README)
• Python: https://datastax.github.io/python-driver/getting_started.html#setting-a-consistency-level

I think it's a no brainer then 😃

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Closed by #149.