java plugin: Implement a reference counter for the JVMEnv pointers.

java plugin: Implement a reference counter for the JVMEnv pointers.

This way one thread entering the Java plugin twice, e. g. with the
following call-chain, will not detach itself from the JVM before it is
completely done with it.

The problematic situation is:
  -> cjni_read
  -> ALLOC JVM
  -> `Read' (in Java)
  -> `DispatchValues' (in Java)
  -> plugin_dispatch_values
  -> cjni_write
  -> ALLOC JVM (this is a no-op!)
  -> `Write' (in Java)
  -> DEALLOC JVM

This last dealloc is prematurely, because the thread is not done with
the JVM yet: It'd like to continue and return from `DispatchValues' to
execute some more Java code..