Make paxos reuse the timestamp generation of normal operation

patch by slebresne; reviewed by iamaleksey for CASSANDRA-7801
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/statements/SelectStatement.java
	src/java/org/apache/cassandra/thrift/CassandraServer.java