Swappable IO layer for MetadataCache.
includes parameterized tests.
Swappable IO layer for ShardState.
stub in a io-datastax module.
Method renames
resolve merge conflicts
Merge branch 'master' into swappable_io
Merge pull request #287 from gdusbabek/swappable_io

Swappable IO for MetadataCache and ShardState* services