Add traffic limiting capabilities

[vysotskylev]

Each access key now corresponds to a TrafficLimiter. has two types of limits:

• Small-scale limit helps to shape the traffic, i.e. to 1 Mbit/sec per key. TCP connections are not closed when limit is exceeded, instead a goroutine waits according to "leaky bucket" algorithm. UDP packets are dropped when the limit is exceeded.
• Large-scale limit restricts how much traffic the given key may use in, say, a month. When the limit is exceeded, TCP connections are closed and UDP packets are dropped.

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[bemasc]

Hi @vysotskylev. Could you explain more about the use case here? As you may know, Outline already supports a rolling 30-day traffic limit. Limiting the instantaneous bandwidth is unlikely to be necessary, as congestion control and flow control ensure reasonably fair division of available bandwidth when the server is under load.

[vysotskylev]

@bemasc Yes, I think, the large-scale limits are redundant. But we find the instantaneous limits very useful in our services (with many keys and connections on a single node), as the "built-in" flow control is far from perfect. Moreover, it seems to fail in case of UDP service (AFAIK). So I've created another PR with just instantaneous rate limits: #119.

This one can be closed.