Matrix bridge: On M_LIMIT_EXCEEDED, matterbridge drops messages instead of backoff+retry

[BenWiederhake]

Describe the bug

When there is a lot of traffic, matterbridge may end up making a lot of requests to the matrix homeserver, which may respond with M_LIMIT_EXCEEDED. Some of these messages are lost, without good indication in the chat itself – in particular, immediately following messages make it through just fine.

To Reproduce

• Set up and start a bridge between Telegram and matrix
• Upload a bunch of JavaScript files to Telegram (Why specifically JS? Because there's a different bug. Now you know how I ran into it.)
• Observe that only some of them make it to Matrix.

Expected behavior

All of the files should eventually "arrive" in Matrix. For example, matterbridge could back off and try again "after a few moments".

Screenshots/debug logs

Not really appliccable. Here's a log line anyway.

time="2020-09-09T23:50:52+02:00" level=error msg="file comment failed: gomatrix.HTTPError{Contents:[]uint8{0x7b, 0x22, 0x65, … some hex …, 0x7d}, WrappedError:gomatrix.RespError{ErrCode:"M_LIMIT_EXCEEDED", Err:"Too Many Requests"}, Message:"Failed to PUT JSON to /_matrix/client/r0/rooms/!SeCrEtRoOm:matrix.org/send/m.room.message/go1234567890123456789", Code:429}" func=handleUploadFile file="bridge/matrix/matrix.go:345" prefix=matrix

Environment (please complete the following information):

• OS: Linux
• Matterbridge version: 1.18.3

[BenWiederhake]

Note that this is different from #1201, as here we encounter a ratelimit on sending, which will require a more involved message queue.

[42wim]

@BenWiederhake care to test PR #1326 ?