Configurable rate limit in DHT server

[WeirdCarrotMonster]

While trying to figure out why my instance suddenly stopped finding new torrents, i've noticed many KRPC error 201: too many requests logs. While this wasn't a source of my problem, i found out that there's no actual way to increase the limit of requests from a single IP.

This PR adds config options for that. Old hard-coded values are moved to defaults.

+---------------------------------------------+---------------------------+-------------------------+-------------------------+---------+
| dht_server.rate_limit:                      | server.RateLimitConfig    |                         |                         |         |
+---------------------------------------------+---------------------------+-------------------------+-------------------------+---------+
| dht_server.rate_limit.burst                 | int                       | 100                     | 4                       | env     |
| dht_server.rate_limit.size                  | int                       | 1000                    | 1000                    | default |
| dht_server.rate_limit.ttl                   | time.Duration             | 20s                     | 20s                     | default |
| dht_server.rate_limit.window_size           | time.Duration             | 1s                      | 1s                      | default |
+---------------------------------------------+---------------------------+-------------------------+-------------------------+---------+