Rename internal classes and interfaces to represent the modern Cassandra terminology

Some of the class names we are using aren't exactly clear to the newcomers. We've already gone through the Table->Keyspace renaming, but it's not enough.

Other things to consider:

• ColumnFamilyStore -> ColumnFamily (non-critical)
• Column -> Cell (where appropriate) done in e50d6af12fabac98e3cb4bd589e2b92212b041f4
• ColumnFamily -> Cells
• Row -> Partition (PartitionFragment?) (where appropriate)
• RowMutation -> Mutation done in 6bbb13b9b0fe62de2a2140055af2ea6968c73ccc