THttpClient:Flush creates an http.Client object every time it is called. From the Go docs regarding the use of these client objects:
"Clients and Transports are safe for concurrent use by multiple goroutines and for efficiency should only be created once and re-used."
I suggest that the client used by the THttpTransport default to using either the http.DefaultClient or a package scoped default client. The transport should also accept a http.Client as an optional parameter so users of the library are free to control this behavior.
With the current implementation I have run into issues when spinning up ~1000 separate instances of the THttpClient for a load test application I have written. Switching to using a single shared http client resolved all connection issues I had.