Improve performance of transport in-use list.
Improve performance of transport in-use list.

Connector keeps its in-use transport in a dictionary `_acquired`
mapping hostnames/ports/ssl to lists of transports.

Applications that open many connections to the same remote web service
end up having a single-item dictionary with a long list as value.

Whenever a transport is released, the item is `.remove()`d from the list
that has a O(n) complexity. Also `.append()` on a python list can have
O(n) complexity.

In this patch we use a set instead, so `.remove()` and `add()` have O(1)

z2v/aiohttp,juliatem/aiohttp,rutsky/aiohttp,singulared/aiohttp,esaezgil/aiohttp,arthurdarcet/aiohttp,esaezgil/aiohttp,elastic-coders/aiohttp,z2v/aiohttp,elastic-coders/aiohttp,hellysmile/aiohttp,KeepSafe/aiohttp,mind1master/aiohttp,jettify/aiohttp,decentfox/aiohttp,jettify/aiohttp,mind1master/aiohttp,decentfox/aiohttp,playpauseandstop/aiohttp,elastic-coders/aiohttp,Eyepea/aiohttp,morgan-del/aiohttp,jashandeep-sohi/aiohttp,avanov/aiohttp,alex-eri/aiohttp-1,moden-py/aiohttp,vaskalas/aiohttp,KeepSafe/aiohttp,hellysmile/aiohttp,vaskalas/aiohttp,jashandeep-sohi/aiohttp,pathcl/aiohttp,vedun/aiohttp,KeepSafe/aiohttp,esaezgil/aiohttp,arthurdarcet/aiohttp,moden-py/aiohttp,AraHaanOrg/aiohttp,vaskalas/aiohttp,vasylbo/aiohttp,jashandeep-sohi/aiohttp,flying-sheep/aiohttp,arthurdarcet/aiohttp,Insoleet/aiohttp,rutsky/aiohttp,noodle-learns-programming/aiohttp,alex-eri/aiohttp-1,panda73111/aiohttp,pfreixes/aiohttp,jettify/aiohttp,pfreixes/aiohttp,z2v/aiohttp,panda73111/aiohttp,juliatem/aiohttp,andyaguiar/aiohttp,mind1master/aiohttp,alex-eri/aiohttp-1,singulared/aiohttp,moden-py/aiohttp,alunduil/aiohttp,decentfox/aiohttp,singulared/aiohttp,Srogozins/aiohttp,danielnelson/aiohttp,AraHaanOrg/aiohttp,rutsky/aiohttp,panda73111/aiohttp,sterwill/aiohttp