refactor sinkResponseFile and add downloadC

Remote.S3 and Remote.Helper.Http both had similar code to sink a
http-conduit Response to a file; refactor out sinkResponseFile.

downloadC downloads an url to a file using http-conduit, and supports
resuming. Falls back to curl to handle urls that http-conduit does not
support. This is not used yet, but the goal is to replace download with
it.

git-annex.cabal: conduit-extra was not actually used for a long time,
remove the dep. conduit moves into the main dependency list, but since
http-conduit was already in there, and it depends on conduit, that's not
really adding a new build dep.

This commit was supported by the NSF-funded DataLad project.

Remote/Helper/Http.hs

@@ -15,6 +15,7 @@ import Utility.Metered
 import Remote.Helper.Special
 import Network.HTTP.Client (RequestBody(..), Response, responseStatus, responseBody, BodyReader, NeedsPopper)
 import Network.HTTP.Types
+import Network.HTTP.Conduit
 
 import qualified Data.ByteString.Lazy as L
 import qualified Data.ByteString as S
@@ -71,15 +72,5 @@ handlePopper numchunks chunksize meterupdate h sink = do
 httpBodyRetriever :: FilePath -> MeterUpdate -> Response BodyReader -> IO ()
 httpBodyRetriever dest meterupdate resp
 	| responseStatus resp /= ok200 = giveup $ show $ responseStatus resp
-	| otherwise = bracket (openBinaryFile dest WriteMode) hClose (go zeroBytesProcessed)
-  where
-	reader = responseBody resp
-	go sofar h = do
-		b <- reader
-		if S.null b
-			then return ()
-			else do
-				let sofar' = addBytesProcessed sofar $ S.length b
-				S.hPut h b
-				meterupdate sofar'
-				go sofar' h
+	| otherwise = runResourceT $
+		sinkResponseFile meterupdate zeroBytesProcessed dest WriteMode resp