David Robertson 7111ab1d9f
Check that the homeserver is reachable at startup
Closes #286.

Tested as follows (see line 4):

```
Sync v3 [0.99.10] (6fe9b18)
Debug=true LogLevel= MaxConns=0
Starting prometheus listener on :6060
13:23:33 WRN Could not contact upstream homeserver. Is SYNCV3_SERVER set correctly? error="Get \"http://localhost:8888/_matrix/client/versions\": dial tcp [::1]:8888: connect: connection refused" dest=http://localhost:8888
2023/09/26 13:23:33 OK   20230728114555_device_data_drop_id.sql (7.48ms)
2023/09/26 13:23:33 OK   20230802121023_device_data_jsonb.go (13.85ms)
2023/09/26 13:23:33 OK   20230814183302_cbor_device_data.go (9.01ms)
2023/09/26 13:23:33 OK   20230822180807_bogus_snapshot_cleanup.go (7.64ms)
2023/09/26 13:23:33 OK   20230913120537_events_missing_previous.sql (7.15ms)
2023/09/26 13:23:33 goose: successfully migrated database to version: 20230913120537
13:23:33 INF creating handler
13:23:33 INF retrieved global snapshot from database
13:23:33 INF listening on 0.0.0.0:8844
13:23:33 INF StartV2Pollers num_devices=0 num_fail_decrypt=0
13:23:33 INF StartV2Pollers finished
```
2023-09-26 13:24:26 +01:00
..
2023-06-26 21:12:56 -07:00
2023-09-21 17:15:23 +01:00
2023-06-19 16:49:37 +01:00
2023-07-27 12:10:46 +01:00