NEXUS-5727: Prevent multiple creation of thread.

Shiro 1.2.1 suffers from a concurrency issue that causes
multiple session validation threads to be lazily
created upon first request for a session. We saw
this on OSO Nexus instance for example, where we
had 5 such threads concurrently sweeping for stopped
sessions.

This applies a workaround usable with Shiro 1.2.1.
The workaround is to "preinstall" the session
validating thread, so no "lazy" creation
(that is not thread safe) will happen.

Issues:
https://issues.sonatype.org/browse/NEXUS-5727
https://issues.apache.org/jira/browse/SHIRO-443
Merge remote-tracking branch 'origin/master' into nexus-5727-session-validation-thread
NEXUS-5727: Simplified the fix.
NEXUS-5727: Expanding module test

Checking for properly set cache too
Merge remote-tracking branch 'origin/master' into nexus-5727-session-validation-thread
Merge remote-tracking branch 'origin/master' into nexus-5727-session-validation-thread
NEXUS-5727: Applied Stuart's advice

Removed the clumsy setSessionDAO method
from fixed classes that were needed only to
"trigger" the member injection.

Using bind.toConstructor works without this.
Merge pull request #7 from sonatype/nexus-5727-session-validation-thread

[REVIEW] NEXUS-5727: Prevent multiple creation of thread.