No longer use COMPACT STORAGE for Cassandra 3+ with CQL backend

[pluradj]

Fixes #1023

Signed-off-by: Jason Plurad pluradj@us.ibm.com

• Added compact storage option for Cassandra 2 and earlier only. Default is to use compact storage when using Cassandra 2 and earlier, maintaining the previous behavior.
• Updated docs to prefer CQL over Thrift going forward.
• Updated to Cassandra 2.1.20.

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[porunov]

LGTM

[sjudeng]

Very nice. All code updates are covered by tests. Also compared new (cql) config files with existing (cassandra) variants and updates were as expected.