move TransferrerPool from assistant

This old code will now be useful for git-annex beyond the assistant.

git-annex won't use the CheckTransferrer part, and won't run transferkeys
as a batch process, and will want withTransferrer to not shut down
transferkeys processes. Still, the rest of this is a good fit for what I
need now.

Also removed some dead code, and simplified a little bit.

This commit was sponsored by Mark Reidenbach on Patreon.

Utility/Batch.hs

@@ -10,6 +10,7 @@
 module Utility.Batch (
 	batch,
 	BatchCommandMaker,
+	nonBatchCommandMaker,
 	getBatchCommandMaker,
 	toBatchCommand,
 	batchCommand,
@@ -50,6 +51,9 @@ batch a = a
  - are available in the path. -}
 type BatchCommandMaker = (String, [CommandParam]) -> (String, [CommandParam])
 
+nonBatchCommandMaker :: BatchCommandMaker
+nonBatchCommandMaker = id
+
 getBatchCommandMaker :: IO BatchCommandMaker
 getBatchCommandMaker = do
 #ifndef mingw32_HOST_OS