[iocp] various fixes

- Properly handle send_error:
    Set proper code and socket structure for its overlapped field

- Handle SO_UPDATE_CONNECT_CONTEXT failures:
    We sometimes get WSAENOTSOCK on this one

- Add WSAENOTSOCK to error list for w32err_2errno

- Fix socket leakage for each socket created (ouch!)

Change-Id: Ice2f451dad1e862fd6800df669b4af9b8c13a81b
Reviewed-on: http://review.couchbase.org/28281
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>