[regression] Increase RESTServer buffer size back to 64k

HBASE-14492 is not longer present in our current releases after HBASE-12894. Unfortunately our RESTServer is not extending HttpServer which means that DEFAULT_MAX_HEADER_SIZE is not being set and HTTP requests with a very large header can still cause connection issues for clients. A quick fix is just to add the settings to the HttpConfiguration configuration object. A long term solution should be to re-factor services that create an HTTP server and normalize all configuration settings across all of them.