don't flush on every body part

[artemredkin]

Right now we call flush on every body part write, this can be suboptimal, closes #203

Motivation:
Library users can write data in small chunk, in this case we are not buffering enough data, better solution would be to buffer at the NIO level in this case.

Modifications:
Body part write is not not flushing, just write

Result:
Body part writes are not flushing anymore

[weissi]

I don't think this achieves quite what we want

[weissi]

hmm, this would now never flush any body which means we'll load the whole body into memory before sending.

I think the right thing to do is:

• if you know that whole body straight away (without streaming), then we should do
  write(.head), .write(.body), .write(end), flush
• if we actually want to stream the body, then
  writeAndFlush(.head), writeAndFlush(.body), writeAndFlush(.end)

[weissi]

We also need a test actually that shows that this PR doesn't work. We should have a test which checks that body chunks do arrive at the other end before the next bit is sent out.

[weissi]

wrote #219 to test this. This PR should go in after #219 to prove that it works.

[ktoso]

Just as a heads up, the main development branch has been changed to main, following the Swift policy on this.

This PR has been re-targeted to main and should just work. However when performing rebases etc please keep this in mind -- you may want to fetch the main branch and rebase onto the main branch from now on since master is not up-to-date anymore and is going to be deleted shortly.