Rewrite cache implementation to cache only on GET (#7694)

Fixes #7458
Fixes #7573 
Fixes #7938 
Fixes #6934
Fixes #6265 
Fixes #6630 

This will allow the cache to consistently work for
server and gateways. Range GET requests will
be cached in the background after the request
is served from the backend.

- All cached content is automatically bitrot protected.

- Avoid ETag verification if a cache-control header
is set and the cached content is still valid.

- This PR changes the cache backend format, and all existing
content will be migrated to the new format. Until the data is
migrated completely, all content will be served from the backend.

cmd/bucket-handlers.go

@@ -106,9 +106,7 @@ func (api objectAPIHandlers) GetBucketLocationHandler(w http.ResponseWriter, r *
 	}
 
 	getBucketInfo := objectAPI.GetBucketInfo
-	if api.CacheAPI() != nil {
-		getBucketInfo = api.CacheAPI().GetBucketInfo
-	}
+
 	if _, err := getBucketInfo(ctx, bucket); err != nil {
 		writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL, guessIsBrowserReq(r))
 		return
@@ -203,9 +201,6 @@ func (api objectAPIHandlers) ListBucketsHandler(w http.ResponseWriter, r *http.R
 	}
 
 	listBuckets := objectAPI.ListBuckets
-	if api.CacheAPI() != nil {
-		listBuckets = api.CacheAPI().ListBuckets
-	}
 
 	if s3Error := checkRequestAuthType(ctx, r, policy.ListAllMyBucketsAction, "", ""); s3Error != ErrNone {
 		writeErrorResponse(ctx, w, errorCodes.ToAPIErr(s3Error), r.URL, guessIsBrowserReq(r))
@@ -747,9 +742,7 @@ func (api objectAPIHandlers) HeadBucketHandler(w http.ResponseWriter, r *http.Re
 	}
 
 	getBucketInfo := objectAPI.GetBucketInfo
-	if api.CacheAPI() != nil {
-		getBucketInfo = api.CacheAPI().GetBucketInfo
-	}
+
 	if _, err := getBucketInfo(ctx, bucket); err != nil {
 		writeErrorResponseHeadersOnly(w, toAPIError(ctx, err))
 		return
@@ -779,9 +772,7 @@ func (api objectAPIHandlers) DeleteBucketHandler(w http.ResponseWriter, r *http.
 	}
 
 	deleteBucket := objectAPI.DeleteBucket
-	if api.CacheAPI() != nil {
-		deleteBucket = api.CacheAPI().DeleteBucket
-	}
+
 	// Attempt to delete bucket.
 	if err := deleteBucket(ctx, bucket); err != nil {
 		writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL, guessIsBrowserReq(r))