replace SSE-C key derivation scheme (#5168)

This chnage replaces the current SSE-C key derivation scheme. The 'old'
scheme derives an unique object encryption key from the client provided key.
This key derivation was not invertible. That means that a client cannot change
its key without changing the object encryption key.
AWS S3 allows users to update there SSE-C keys by executing a SSE-C COPY with
source == destination. AWS probably updates just the metadata (which is a very
cheap operation). The old key derivation scheme would require a complete copy
of the object because the minio server would not be able to derive the same
object encryption key from a different client provided key (without breaking
the crypto. hash function).

This change makes the key derivation invertible.

cmd/generic-handlers_test.go

@@ -157,11 +157,11 @@ var containsReservedMetadataTests = []struct {
 		shouldFail: true,
 	},
 	{
-		header:     http.Header{ServerSideEncryptionKDF: []string{SSEKeyDerivationHmacSha256}},
+		header:     http.Header{ServerSideEncryptionSealAlgorithm: []string{SSESealAlgorithmDareSha256}},
 		shouldFail: true,
 	},
 	{
-		header:     http.Header{ServerSideEncryptionKeyMAC: []string{"mac"}},
+		header:     http.Header{ServerSideEncryptionSealedKey: []string{"mac"}},
 		shouldFail: true,
 	},
 	{