Reduce allocations and copies in pgproto3

Altered chunkreader to never reuse memory.

Altered pgproto3 to to copy memory when decoding. Renamed UnmarshalBinary to
Decode because of changed semantics.

pgproto3.go

@@ -2,8 +2,13 @@ package pgproto3
 
 import "fmt"
 
+// Message is the interface implemented by an object that can decode and encode
+// a particular PostgreSQL message.
+//
+// Decode is allowed and expected to retain a reference to data after
+// returning (unlike encoding.BinaryUnmarshaler).
 type Message interface {
-	UnmarshalBinary(data []byte) error
+	Decode(data []byte) error
 	MarshalBinary() (data []byte, err error)
 }
 
@@ -17,58 +22,6 @@ type BackendMessage interface {
 	Backend() // no-op method to distinguish frontend from backend methods
 }
 
-// func ParseBackend(typeByte byte, body []byte) (BackendMessage, error) {
-// 	switch typeByte {
-// 	case '1':
-// 		return ParseParseComplete(body)
-// 	case '2':
-// 		return ParseBindComplete(body)
-// 	case 'C':
-// 		return ParseCommandComplete(body)
-// 	case 'D':
-// 		return ParseDataRow(body)
-// 	case 'E':
-// 		return ParseErrorResponse(body)
-// 	case 'K':
-// 		return ParseBackendKeyData(body)
-// 	case 'R':
-// 		return ParseAuthentication(body)
-// 	case 'S':
-// 		return ParseParameterStatus(body)
-// 	case 'T':
-// 		return ParseRowDescription(body)
-// 	case 't':
-// 		return ParseParameterDescription(body)
-// 	case 'Z':
-// 		return ParseReadyForQuery(body)
-// 	default:
-// 		return ParseUnknownMessage(typeByte, body)
-// 	}
-// }
-
-// func ParseFrontend(typeByte byte, body []byte) (FrontendMessage, error) {
-// 	switch typeByte {
-// 	case 'B':
-// 		return ParseBind(body)
-// 	case 'D':
-// 		return ParseDescribe(body)
-// 	case 'E':
-// 		return ParseExecute(body)
-// 	case 'P':
-// 		return ParseParse(body)
-// 	case 'p':
-// 		return ParsePasswordMessage(body)
-// 	case 'Q':
-// 		return ParseQuery(body)
-// 	case 'S':
-// 		return ParseSync(body)
-// 	case 'X':
-// 		return ParseTerminate(body)
-// 	default:
-// 		return ParseUnknownMessage(typeByte, body)
-// 	}
-// }
-
 type invalidMessageLenErr struct {
 	messageType string
 	expectedLen int