Limit POST form fields and file size + Generic Request Size limiter (#2317)

* Use less memory when receiving a file via multipart
* Add generic http request maximum size limiter to secure against malicious clients

typed-errors.go

@@ -32,3 +32,6 @@ var errInvalidToken = errors.New("Invalid token")
 
 // If x-amz-content-sha256 header value mismatches with what we calculate.
 var errContentSHA256Mismatch = errors.New("sha256 mismatch")
+
+// used when we deal with data larger than expected
+var errSizeUnexpected = errors.New("data size larger than expected")