fix: exchange: allow up to 10k messages per block

Also explicitly limit how many bytes we're willing to read in one go
such that we're capable of reading a worst-case tipset (like, really,
never going to happen worst-case). Previously, this wasn't an issue.
However, we've bumped the max number of messages from 8,192 to 150,000
and need to limit allocations somewhere else.

gen/main.go

@@ -92,7 +92,7 @@ func main() {
 	err = gen.WriteTupleEncodersToFile("./chain/exchange/cbor_gen.go", "exchange",
 		exchange.Request{},
 		exchange.Response{},
-		exchange.CompactedMessages{},
+		exchange.CompactedMessagesCBOR{},
 		exchange.BSTipSet{},
 	)
 	if err != nil {