Use less cpu

Hey!

I've added a small fix to make this run smoother on embedded systems like Raspberry Pi or the Beaglebone Black (both ARM systems).
I had CPU usage at about 25% without any connection to websockets on an ARM cortex A8 at 1GHz.

I added an property to tcpServer.cs that allows to set an max rate of new connections per second.
when i now set this to 5 on the ARM A8 it dropped to less then 2%.

-Markus