draft for streaming el fixes

[artemredkin]

Fixes programming model for upload streaming

Motivation:
Right now EL on which we execute streaming call back is not explicitly defined. Also, we do not check on which EL part write is executed, meaning we can fail if .write is not called on correct EL.

Modifications:

1. Call to body.stream is now executed on tasks EL
2. Calls to writeAndFlush are always called on channel EL
3. Added tests for those to cases
4. Removed a hack in test introduced in test: that streaming actually works #219
5. Fixed test: that streaming actually works #219 shutdown to not call to shutdown EL

Result:

1. closes programming model: EventLoopPreference.delegate(on:) not respected for .stream #198
2. closes StreamWriter expects to be called on some internal EventLoop #200

[weissi]

Thank you! LGTM!