fix various HTTP response codes

[mastercactapus]

Description:
Fixes various incorrect status code responses

• Hitting a limit (like too many unacked alerts) now correctly responds with 409 Conflict fixing a regression.
• Body size limit errors now respond with 413 Content Too Large
• Aborted/disconnected requests will be sent a 499 to distinguish from 400
• Requests in the queue that timeout will now get 408 Request Timeout (as opposed to the 429 Too Many Requests when the queue is full)

Also a test was added for the body size limit, as prior to this change they would incorrectly result in 500 errors.