Add basic stream control to sync streams

[nick4fake]

Hey,

There is always a chance that input and output streams have different bandwidth. Like if we are reading a file and sending it somewhere it is read at much higher speed. Result is simple - more and more data is buffered into memory.

Possible solution could be usage of buffered stream, that pauses itself when buffer overflows + "drain" listener on output stream, that resumes it.

[clue]

Yes, please 👍

Afaict this will be addressed as part of #62 because the newer react/http-client versions should actually have proper streaming behavior.

[clue]

Small progress update: Now that #72 is in, we support react/http-client v0.5 which has proper stream support (https://github.com/reactphp/http-client/releases).

I'll update the dependencies to enforce this minimum version soon, so that we can rely on this feature in our test suite 👍