Fix #75: Support async requests for tomcat7

Set asyncSupported(true) on valves in tc7 msm.
Merge PR #16: Make MemcachedBackupSessionTest.testRefCount more robust

PR text:
This test can fail at line 65 because sometimes the same thread will be used in the pool and so the reference is not incremented.  I don't know exactly what this means for the semantics involved.