TS: removed half-closing client socket after sending request.

Not allowed by HTTP spec as "end of reqquest" delimiter.
TS: making the PeriodicDownload "small request" assumption explicit

And therefor possibly blocking i.s.o. sending an incomplete request.
TS: replaced sendall "sync"-hack with a "quickOrAsyncSend".

This tries sok.send(data) without checking readability (thus use wisely!),
and if not all data is send immidiately, fallback to reactor async
writing.

This way, no unexpected long blocking calls can occur, while still being
able to "be quick about it" when possible.
TS: tested the need async support in _quickOrAsyncSend (whitebox).
TS: tested not shutting down TCP socket connection "write half" too soon (whiteboxed).