[http] Don't treat graceful close as an error

Now that we're sending Connection: close, it's perfectly fine (and in
fact, anticipated) that the server should close the connection when it's
ready. In this case, we need to consider it a success iff the parser has
finished too.

- Added LCB_SOCKRW_SHUTDOWN (technically, 0 means one of the two streams
  has been closed, so it's not technically a close in TCP parlance)

Change-Id: I126565c50cf3dbe9bc2ba79445bc8fd80e85962c
Reviewed-on: http://review.couchbase.org/28285
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>