Allow alter keyspace on system_traces

patch by yukim; reviewed by jbellis for CASSANDRA-6016
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	src/java/org/apache/cassandra/service/CassandraDaemon.java
Tracing should log write failure rather than raw exceptions
patch by jbellis; reviewed by ayeschenko for CASSANDRA-6133
merge from 1.2
Merge branch 'cassandra-2.0' into trunk