Retry initial database connection until it comes up

[dongresource]

TODO:

• Don't hide the full error message (.with_parent(e)) when it's something other than a typical connection failure
• Add a config value for the retry grace period
• Check for any race conditions between DB reconnection and player evacuation when a shard-to-DB connection breaks
• Change log severity from Fatal to Warning (or skip printing the log line, to match Login<->Shard reconnect?)