chunker: finish meta-format before release

changes:
- chunker: remove GetTier and SetTier
- remove wdmrcompat metaformat
- remove fastopen strategy
- make hash_type option non-advanced
- adverise hash support when possible
- add metadata field "ver", run strict checks
- describe internal behavior in comments
- improve documentation

note:
wdmrcompat used to write file name in the metadata, so maximum metadata
size was 1K; removing it allows to cap size by 200 bytes now.

backend/chunker/chunker_test.go

@@ -28,10 +28,14 @@ var (
 // dynamic chunker overlay wrapping a local temporary directory.
 func TestIntegration(t *testing.T) {
 	opt := fstests.Opt{
-		RemoteName:                   *fstest.RemoteName,
-		NilObject:                    (*chunker.Object)(nil),
-		SkipBadWindowsCharacters:     !*UseBadChars,
-		UnimplementableObjectMethods: []string{"MimeType"},
+		RemoteName:               *fstest.RemoteName,
+		NilObject:                (*chunker.Object)(nil),
+		SkipBadWindowsCharacters: !*UseBadChars,
+		UnimplementableObjectMethods: []string{
+			"MimeType",
+			"GetTier",
+			"SetTier",
+		},
 		UnimplementableFsMethods: []string{
 			"PublicLink",
 			"OpenWriterAt",