dropping requests to .well-known leads to slow media federation

This issue has been migrated from [#7231](https://github.com/matrix-org/synapse/issues/7231).

---

### Description

When a server has it's firewall set to drop incoming requests to ports it doesn't use (pf default for the block rule) and doesn't use .well-known (so it drops requests to 443, specifically `https://server:443/.well-known/matrix/server`), every media request needs to wait for the .well-known request to timeout, before it actually starts fetching the media. So every image in a shared room takes around 30 seconds to load over federation.

### Steps to reproduce

- set up a server A, that only runs on port 8448
- set the firewall of server A to drop requests on port 443 (use SRV or default 8448 for federation setup)
- set up server B
- share a room between server A and B
- let server A send an image

Server B will now wait for 30 seconds on the .well-known timeout, before it sends the actual media request. This happens for every media!

### Expected behaviour

Server B caches the .well-known result and only waits for 30 seconds on the first timeout. Maybe it is just the media worker, that doesn't do that.

relevant logs:

```
2020-04-05 21:39:01,516 - synapse.http.matrixfederationclient - 408 - INFO - GET-11022 - {GET-O-345} [pink.packageloss.eu] Sending request: GET matrix://pink.packageloss.eu/_matrix/media/v1/download/pink.packageloss.eu/3d1a80ed7d2b89457d96e5212f2378a958b7560c?allow_remote=false; timeout 60.000000s
2020-04-05 21:39:01,517 - synapse.http.federation.well_known_resolver - 234 - INFO - GET-11022 - Fetching https://pink.packageloss.eu/.well-known/matrix/server
2020-04-05 21:39:03,892 - synapse.access.http.8085 - 302 - INFO - GET-11023 - 127.0.0.1 - 8085 - {None} Processed request: 0.003sec/-0.000sec (0.002sec, 0.000sec) (0.001sec/0.001sec/1) 6756B 200 "GET /_matrix/media/r0/download/pink.packageloss.eu/b195fb40cab5095adf1551b876d99610bad24f60 HTTP/1.0" "mtxclient v0.3.0" [0 dbevts]
2020-04-05 21:39:31,825 - synapse.http.federation.well_known_resolver - 250 - INFO - GET-11022 - Error fetching https://pink.packageloss.eu/.well-known/matrix/server: User timeout caused connection failure.
2020-04-05 21:39:31,828 - synapse.http.federation.matrix_federation_agent - 242 - INFO - GET-11022 - Connecting to pink.packageloss.eu:8448
2020-04-05 21:39:32,211 - synapse.http.matrixfederationclient - 442 - INFO - GET-11022 - {GET-O-345} [pink.packageloss.eu] Got response headers: 200 OK
2020-04-05 21:39:32,212 - synapse.http.matrixfederationclient - 909 - INFO - GET-11022 - {GET-O-345} [pink.packageloss.eu] Completed: 200 OK [14490 bytes]
2020-04-05 21:39:32,212 - synapse.rest.media.v1.media_repository - 406 - INFO - GET-11022 - Stored remote media in file '/var/lib/synapse/media_store/remote_content/pink.packageloss.eu/td/yY/oLPpEPNArfeZksvNvGWv'
2020-04-05 21:39:32,356 - synapse.access.http.8085 - 302 - INFO - GET-11022 - ::1 - 8085 - {None} Processed request: 30.857sec/-0.000sec (0.047sec, 0.001sec) (0.003sec/0.110sec/7) 14490B 200 "GET /_matrix/media/r0/download/pink.packageloss.eu/3d1a80ed7d2b89457d96e5212f2378a958b7560c HTTP/1.0" "mtxclient v0.3.0" [0 dbevts]
```
### Version information

- **Homeserver**: pink.packageloss.eu (server A) and neko.dev (server B)

If not matrix.org:

- **Version**: 10.x and 12.3

- **Install method**: ports/ebuild

- **Platform**: FreeBSD/Gentoo