Add ScyllaDB driver extensions

[dorlaor]

Scylla enhances the CQL protocol with these two capabilities:

• Cache hint
  Simply flag a query to BYPASS the cache. This way queries that
  just scan do not need to be kept in the database cache. That's trivial
  to implement

• Shard awareness
  Scylla provides per-core topology information, this way the driver can
  send the query to the core that handles the range. It improves load balancing
  and reduces a shard hop. It's implemented by Python, Java, go, C++ drivers.

[whatyouhide]

Hey @dorlaor, thanks for the report! Those features sound great but @lexmag and I have no bandwidth right now to work on anything here. If you have consistent time to work on this and would be able to drive it to conclusion, I'd be happy to provide support, but otherwise we won't get these in for the time being :(

[dorlaor]

Once we'll have an Elixir fan in the neighborhood we'll point him/her towards this, sure

[whatyouhide]

BYPASS CACHE is a CQL-level functionality, so it requires no changes to the driver. I added a test to make sure we support it in e7c9143.

As for shard awareness, I opened #324 to track it. Closing this down then!