Duplicacy should respect 429 responses (Dropbox)

Please describe what you are doing to trigger the bug:
Backup or copy with multiple threads

Please describe what you expect to happen (but doesn’t):
The backup or copy should succeed,

Please describe what actually happens (the wrong behaviour):
The backup fails with a “too many write operations” error.

Dropbox will respond with a 429 response code and Retry-After header. Sleeping for that many of seconds and retrying will allow the operation to succeed.

I’ve sent a pull request with a very simple fix. I’ve been using it for a few days and it will pause and resume for 429 errors.

It could be better when working with multiple threads by disallowing simultaneous writers, but for now it will prevent a Too Many Requests error from causing a failure.