Allow synchronous replication if enabled. (#11165)

Synchronous replication can be enabled by setting the --sync
flag while adding a remote replication target.

This PR also adds proxying on GET/HEAD to another node in a
active-active replication setup in the event of a 404 on the current node.

cmd/object-api-options.go

@@ -66,6 +66,7 @@ func getDefaultOpts(header http.Header, copySource bool, metadata map[string]str
 	if crypto.S3.IsRequested(header) || (metadata != nil && crypto.S3.IsEncrypted(metadata)) {
 		opts.ServerSideEncryption = encrypt.NewSSE()
 	}
+	opts.ProxyRequest = header.Get(xhttp.MinIOSourceProxyRequest) == "true"
 	return
 }