ACCUMULO-2785 Create a random string in the session, and provide it in requests to mitigate CSRF.
Merge branch '1.5.2-SNAPSHOT' into 1.6.1-SNAPSHOT

Conflicts:
	server/monitor/src/main/java/org/apache/accumulo/monitor/servlets/ShellServlet.java
Merge branch '1.6.1-SNAPSHOT'