Tulip issue #193: Convert StreamWriter.drain() to a classic coroutine
Tulip issue #193: Convert StreamWriter.drain() to a classic coroutine

Replace also _make_drain_waiter() function with a classic _drain_helper()
coroutine.

jashandeep-sohi/asyncio,manipopopo/asyncio,gsb-eng/asyncio,ajdavis/asyncio,manipopopo/asyncio,gvanrossum/asyncio,haypo/trollius,haypo/trollius,fallen/asyncio,jashandeep-sohi/asyncio,gvanrossum/asyncio,Martiusweb/asyncio,vxgmichel/asyncio,gsb-eng/asyncio,1st1/asyncio,1st1/asyncio,jashandeep-sohi/asyncio,1st1/asyncio,manipopopo/asyncio,vxgmichel/asyncio,haypo/trollius,fallen/asyncio,fallen/asyncio,Martiusweb/asyncio,ajdavis/asyncio,gvanrossum/asyncio,gsb-eng/asyncio,Martiusweb/asyncio,vxgmichel/asyncio,ajdavis/asyncio