new protocol for transferkeys, with message serialization

Necessarily threw out the old protocol, so if an old git-annex assistant
is running, and starts a transferkeys from the new git-annex, it would
fail. But, that seems unlikely; the assistant starts up transferkeys
processes and then keeps them running. Still, may need to test that
scenario.

The new protocol is simple read/show and looks like this:

TransferRequest Download (Right "origin") (Key {keyName = "f8f8766a836fb6120abf4d5328ce8761404e437529e997aaa0363bdd4fecd7bb", keyVariety = SHA2Key (HashSize 256) (HasExt True), keySize = Just 30, keyMtime = Nothing, keyChunkSize = Nothing, keyChunkNum = Nothing}) (AssociatedFile (Just "foo"))
TransferOutput (ProgressMeter (Just 30) (MeterState {meterBytesProcessed = BytesProcessed 0, meterTimeStamp = 1.6070268727892535e9}) (MeterState {meterBytesProcessed = BytesProcessed 30, meterTimeStamp = 1.6070268728043e9}))
TransferOutput (OutputMessage "(checksum...) ")
TransferResult True

Granted, this is not optimally fast, but it seems good enough, and is
probably nearly as fast as the old protocol anyhow.

emitSerializedOutput for ProgressMeter is not yet implemented. It needs
to somehow start or update a progress meter. There may need to be a new
message that allocates a progress meter, and then have ProgressMeter
update it.

This commit was sponsored by Ethan Aubin

Types/Messages.hs

@@ -19,8 +19,7 @@ data OutputType
 	= NormalOutput
 	| QuietOutput
 	| JSONOutput JSONOptions
-	| SerializedOutput
-	deriving (Show)
+	| SerializedOutput (SerializedOutput -> IO ())
 
 data JSONOptions = JSONOptions
 	{ jsonProgress :: Bool