Emit _RequestHeadersSent internal event

In #311 we introduced _ResponseHeadersSent. So we should also have a _RequestHeadersSent for the sake of symmetry.

Our other events don’t include the word "Headers", e.g. ResponseReceived, TrailersSent.

While we’re doing this refactoring, let’s drop the "Headers", i.e. rename _ResponseHeadersSent to _ResponseSent, and add a new event _RequestSent.