tlv+hop: contain odd/even logic in payload parsing

Tlv is used more widely in lnd than just for the onion payload. This
commit isolated the protocol-specific odd/even logic, so that tlv can be
used freely elsewhere. An example of this use is db serialization.

tlv/record.go

@@ -12,8 +12,9 @@ import (
 // Type is an 64-bit identifier for a TLV Record.
 type Type uint64
 
-// TypeSet is an unordered set of Types.
-type TypeSet map[Type]struct{}
+// TypeSet is an unordered set of Types. The map item boolean values indicate
+// whether the type that we parsed was known.
+type TypeSet map[Type]bool
 
 // Encoder is a signature for methods that can encode TLV values. An error
 // should be returned if the Encoder cannot support the underlying type of val.