Fix HTTP and HTTP2 stats

There are a number of problems with HTTP and HTTP2 stats. Some were introduced with TS-3612. Other problems appear to predate those changes.

proxy.process.http.current_client_transactions was not being adequately decremented. With this fix it now corresponds to the number of active HttpSMs.
proxy.process.http2.current_client_sessions wasn't being set accurately.
proxy.process.http2.total_client_streams wasn't being set at all.

Originally had the Http2 logic adjusting proxy.process.http.current_client_connections and proxy.process.http.total_client_connections. But decided to pull that out and just have it adjust the http2 versions of those stats. So proxy.process.http connection stats just refer to HTTP/1.x protocols (and SPDY) and proxy.process.http2 connection stats refer to HTTP/2 protocols. This gives monitoring and analysis tools the option to look at connections per protocol.