TS-1058: add TSHttpTxnCloseAfterResponse experimental API

Add a new API to force the server end of a HTTP keepalive. This is
useful for a plugin that sets the error body to easuly intercept a
transaction, but needs to ensure that the subsequent request does
not go to a different replica. One example of this use case is a
challenge-response authentication protocol where the challenge data
is held in memory on the session.
TS-1733 Changing it back so that 'C' extension is recognized as a C++ file.

I originally changed this to handle different cases, but James Peach pointed
out on the mailing list that this is traditionally a C++ extension.