JNI: add example threaded client/server applications (JNI only)

[cconlon]

This PR adds two new JNI-level (not JSSE) examples, a threaded client and server example. The client does session resumption when possible and maintains an application-wide client session cache to hold native WOLFSSL_SESSION pointers.

• examples/SimpleThreadedClient.java - makes concurrent client connections to a server located at localhost:11111. Default number of client threads is 5, but can be changed using the -n <num_connections> command line argument. This example implements a simple application-wide Java client cache where native WOLFSSL_SESSION pointers are stored and used for session resumption where possible. See code comments for further explanation.
• examples/SimpleThreadedServer.java - starts at localhost:11111 and waits for client connections. When a client connection is received, it is handled in a separate thread.

ZD 17211