[PATCH] Fix JavaHL crash in TunnelAgent.CloseTunnelCallback after GC

Please find test snippet and patch attached.

[[[
Fix JavaHL crash in TunnelAgent.CloseTunnelCallback after GC

When jobject reference is kept across different JNI calls, a new global
reference must be requested with NewGlobalRef(). Otherwise, GC is free
to remove the object. Even if Java code keeps a reference to the object,
GC can still move the object around, invalidating the kept jobject,
which results in a native crash when trying to access it.

[in subversion/bindings/javahl]
* native/OperationContext.cpp
   (OperationContext::openTunnel): Add NewGlobalRef() for kept jobject.
   (OperationContext::closeTunnel): Add a matching DeleteGlobalRef().
]]]