incremental checksum on download from ssh or p2p

Checksum as content is received from a remote git-annex repository, rather
than doing it in a second pass.

Not tested at all yet, but I imagine it will work!

Not implemented for any special remotes, and also not implemented for
copies from local remotes. It may be that, for local remotes, it will
suffice to use rsync, rely on its checksumming, and simply return Verified.
(It would still make a checksumming pass when cp is used for COW, I guess.)

P2P/Protocol.hs

@@ -2,7 +2,7 @@
  -
  - See doc/design/p2p_protocol.mdwn
  -
- - Copyright 2016-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2016-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -18,6 +18,7 @@ import Types (Annex)
 import Types.Key
 import Types.UUID
 import Types.Remote (Verification(..), unVerified)
+import Types.Backend (IncrementalVerifier(..))
 import Utility.AuthToken
 import Utility.Applicative
 import Utility.PartialPrelude
@@ -266,7 +267,7 @@ data LocalF c
 	-- Note: The ByteString may not contain the entire remaining content
 	-- of the key. Only once the temp file size == Len has the whole
 	-- content been transferred.
-	| StoreContentTo FilePath Offset Len (Proto L.ByteString) (Proto (Maybe Validity)) ((Bool, Verification) -> c)
+	| StoreContentTo FilePath (Maybe IncrementalVerifier) Offset Len (Proto L.ByteString) (Proto (Maybe Validity)) ((Bool, Verification) -> c)
 	-- ^ Like StoreContent, but stores the content to a temp file.
 	| SetPresent Key UUID c
 	| CheckContentPresent Key (Bool -> c)
@@ -351,13 +352,13 @@ remove key = do
 	net $ sendMessage (REMOVE key)
 	checkSuccess
 
-get :: FilePath -> Key -> AssociatedFile -> Meter -> MeterUpdate -> Proto (Bool, Verification)
-get dest key af m p = 
+get :: FilePath -> Key -> Maybe IncrementalVerifier -> AssociatedFile -> Meter -> MeterUpdate -> Proto (Bool, Verification)
+get dest key iv af m p = 
 	receiveContent (Just m) p sizer storer $ \offset ->
 		GET offset (ProtoAssociatedFile af) key
   where
 	sizer = fileSize dest
-	storer = storeContentTo dest
+	storer = storeContentTo dest iv
 
 put :: Key -> AssociatedFile -> MeterUpdate -> Proto Bool
 put key af p = do