http: use closeWrapper instead of releaseWrapper

[0intro]

Pooly opens HTTP connections with keep-alive disabled,
but tries to reuse open connections after an HTTP response
has been received. This lead the HTTP server to reset
the TCP connection.

This change implements a closeWrapper, analog to releaseWrapper.
However, closeWrapper closes the TCP connection, instead of
putting it back to the connection pool. Now, NewHTTPTransport
can use closeWrapper instead of releaseWrapper.

[3XX0]

This kind of defeat the purpose of Pooly (i.e. having a pool of connection).
Disabling the keep-alive is needed otherwise the HTTP standard library won't call Close.
On the other hand it looks like they set the close header as well: https://golang.org/src/net/http/server.go#L992

I guess we need to leverage the Hijacker interface