Make new jetty-session-tests module to concentrate all reusable session clustering test code

There are many standard and repeatable tests that we want to be able to apply to the various jetty session clustering implementations which to date include terracotta, openspaces and jdbc. These tests are presently in the jetty module and built into a test artifact, which only gets built as I discovered, if you execute the tests as part of the install goal. The terracotta session implementation does not presently use these tests, but has another set of its own.

We need to make a new jetty-session-tests module and move all of the reusable test code into it. As the terracotta session test design is simpler to understand, we should base this new module on that code, and refactor it to make the session implementation pluggable. We should also add any tests that are necessary to ensure that we have as good or better test coverage than we had with the tests in AbstractSessionTest in the jetty module.

Then, all of the clustered session implementations must be made to depend on the jetty-session-tests artifact.

One difficulty is that the JDBCSessionManager implementation in the jetty module would need to depend on the jetty-session-tests artifact, which itself will have explicit and transitive dependencies on the jetty module, thus creating circular dependencies.