Fix AbstractRowResolver and RowDigestResolver for speculative retries

patch by Jonathan Ellis; reviewed by Aleksey Yeschenko for
CASSANDRA-6194
Merge branch 'cassandra-2.0' into trunk
Revert "Revert use of speculative retries by default for now"

This reverts commit 66fe5bdaaa1f5e22d33528c3d2313cc23e7621d8.
Merge branch 'cassandra-2.0' into trunk
Replace fragile-ish ABSC w/ TMBSC in TraceState
Merge branch 'cassandra-2.0' into trunk