Make sure that the Java callback object won't get garbage-collected while
an RA session is still using it.

[in subversion/bindings/javahl]
* native/Prompter.h (Prompter::Prompter): Make copy constructor private.
  (Prompter::makeCPrompter): Declare a copying factory
* native/Prompter.cpp (Prompter::makeCPrompter): Implement the copying
   factory so that it creates a new global reference to the Java prompter.
* native/SVNClient.cpp (SVNClient::openRemoteSession): Use the copying factory.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1558015 13f79535-47bb-0310-9956-ffa450edef68