Optimization pass for better garbage collection

[cconlon]

This PR tries to do some optimization for better garbage collection, based on testing with VisuaVM.

• Close WolfSSLInputStream/WolfSSLOutputStream when SSLSocket.close() is called. This releases the internal references to WolfSSLSession and WolfSSLSocket to allow easier garbage collection of these Input/OutputStream objects.
• Release/reset WolfSSLEngineHelper state when WolfSSLSocket.close() is called, removing internal object references and allowing easier garbage collection.
• Set clientSNIRequested and ctx to null inside WolfSSLSession when freeSSL() is called, potentially reducing the size of WolfSSLSession objects in memory until they are garbage collected.
• Remove finalizer method from WolfSSLInternalVerifyCb class. This is not needed and slows down garbage collection, since objects with finalizers are put in a different queue.
• Add close() to WolfSSLServerSocket which will close the underlying WolfSSLSocket if called and free up resources earlier than at garbage collection time.

This PR also refactors WolfSSLSession.useALPN() at the JNI level to better guarantee ALPN list is null terminated.