Tightens up the handling of segment cancelations. Also modified the ConcurrentMDXTest so that it behaves a bit more realistically.

Conflicts:

	testsrc/main/mondrian/test/ConcurrentMdxTest.java
Woopsie.
Wraps the server's connection and statement maps into Collections.synchronizedMap. I was able to deadlock these maps easily by bombarding the server with queries using ConcurrentMdxTest.testFlushingDoesNotCauseDeadlock. This will prevent obvious deadlocks.

Conflicts:

	src/main/mondrian/server/MondrianServerImpl.java