Revert "Revert "add single-CF specialization of RowMutation constructor""

This reverts commit ecaa6cd1b74b8caeec03e505f04d676414bb633d.
make RowMutation mutable again in SystemTable.serializeSchema
cli: Warn about missing CQL3 tables in schema descriptions

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-5309
Merge branch 'cassandra-1.2' into trunk