Use LRU for Connection Management

[fhessel]

Is your feature request related to a problem? Please describe.

The server can handle only very few connections in parallel. Connections have a keep-alive timeout of 20 seconds. If incoming connections are queuing up, idling keep-alive connections slow down their acceptance.

Describe the solution you'd like

Idling connections should be dropped based on an LRU mechanism. Incoming connections should drop the least recently used idling connection to get a free connection slot. To assure that an adversary cannot easily drain the active connections, there should also be a limit of connections per Client IP.

Additional context

• see #75