merge 'TCP hardlimits'

Several bugs fixed:
- TCP client count can be limited by the config variable `max_tcp_clients` in `system` (so there are free descriptors for autonomous operation)
- Aggressive limits when client connects, but doesn't start sending data in time
- Throttling doesn't affect activity other then `accept()`
- Possible resource leak on `tcp_sweep`
- Bad logic (started reading from socket before checking for error activity first)
- Bad logic (could close master socket on fd exhaustion)

fixes #326 in 1.6