the delegate's `didSendRequestPart` is invoked at the wrong point in time

[weissi]

Since version 1.6.0 (it worked in 1.5.1), AHC seemingly invokes the delegates didSendRequestPart method when the write of the body part is started and not when the write has succeeded.

That's pretty bad because a user might trigger the generation of the next data to send in this delegate method which therefore might break backpressure.

[weissi]

see also: https://github.com/vapor/vapor/pull/2787/files#diff-818067aa40c78c82b6316d581775acf5a7375ec04ce720511ede7dce613a57a5R830

[Lukasa]

Yup, this definitely isn't right. We could chain the backpressure either off the write promise or the same backpressure future we're using for the StreamWriter