Do verification of checksums of annex objects downloaded from remotes.

* When annex objects are received into git repositories, their checksums are
  verified then too.
* To get the old, faster, behavior of not verifying checksums, set
  annex.verify=false, or remote.<name>.annex-verify=false.
* setkey, rekey: These commands also now verify that the provided file
  matches the key, unless annex.verify=false.
* reinject: Already verified content; this can now be disabled by
  setting annex.verify=false.

recvkey and reinject already did verification, so removed now duplicate
code from them. fsck still does its own verification, which is ok since it
does not use getViaTmp, so verification doesn't happen twice when using fsck
--from.

Command/TestRemote.hs

@@ -159,7 +159,7 @@ test st r k =
 		Just b -> case verifyKeyContent b of
 			Nothing -> return True
 			Just verifier -> verifier k (key2file k)
-	get = getViaTmp k $ \dest ->
+	get = getViaTmp (RemoteVerify r) k $ \dest ->
 		Remote.retrieveKeyFile r k Nothing dest nullMeterUpdate
 	store = Remote.storeKey r k Nothing nullMeterUpdate
 	remove = Remote.removeKey r k
@@ -173,10 +173,10 @@ testUnavailable st r k =
 	, check (`notElem` [Right True, Right False]) "checkPresent" $
 		Remote.checkPresent r k
 	, check (== Right False) "retrieveKeyFile" $
-		getViaTmp k $ \dest ->
+		getViaTmp (RemoteVerify r) k $ \dest ->
 			Remote.retrieveKeyFile r k Nothing dest nullMeterUpdate
 	, check (== Right False) "retrieveKeyFileCheap" $
-		getViaTmp k $ \dest ->
+		getViaTmp (RemoteVerify r) k $ \dest ->
 			Remote.retrieveKeyFileCheap r k Nothing dest
 	]
   where