Explicitly use Long.MAX_VALUE timestamp for counter deletions

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-7346
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql/AbstractModification.java
	src/java/org/apache/cassandra/cql/DeleteStatement.java
	src/java/org/apache/cassandra/cql/UpdateStatement.java