ListObjects Metadata Caching (#10648)

Design: https://gist.github.com/klauspost/025c09b48ed4a1293c917cecfabdf21c

Gist of improvements:

* Cross-server caching and listing will use the same data across servers and requests.
* Lists can be arbitrarily resumed at a constant speed.
* Metadata for all files scanned is stored for streaming retrieval.
* The existing bloom filters controlled by the crawler is used for validating caches.
* Concurrent requests for the same data (or parts of it) will not spawn additional walkers.
* Listing a subdirectory of an existing recursive cache will use the cache.
* All listing operations are fully streamable so the number of objects in a bucket no 
  longer dictates the amount of memory.
* Listings can be handled by any server within the cluster.
* Caches are cleaned up when out of date or superseded by a more recent one.

cmd/object-api-errors.go

@@ -169,6 +169,11 @@ func (e InsufficientReadQuorum) Error() string {
 	return "Storage resources are insufficient for the read operation."
 }
 
+// Unwrap the error.
+func (e InsufficientReadQuorum) Unwrap() error {
+	return errErasureReadQuorum
+}
+
 // InsufficientWriteQuorum storage cannot satisfy quorum for write operation.
 type InsufficientWriteQuorum struct{}
 
@@ -176,6 +181,11 @@ func (e InsufficientWriteQuorum) Error() string {
 	return "Storage resources are insufficient for the write operation."
 }
 
+// Unwrap the error.
+func (e InsufficientWriteQuorum) Unwrap() error {
+	return errErasureWriteQuorum
+}
+
 // GenericError - generic object layer error.
 type GenericError struct {
 	Bucket    string