RATE_LIMITER: protocol to define max throughput / sender / time unit

This is a new protocol which limits the rate of sending messages. Properties max_bytes defines the max number of bytes and time_period the max number of milliseconds. Together, these props define an upper limit of bytes to be sent / time_period, for example
max_bytes="120KB" time_period="3000" defines that we can send a max of 120'000 bytes distributed over the course of 3 seconds.

max_bytes="10MB" time_period="1000" means that a sender can send only 10MB of data per second. If we have 4 senders, then the total throughtput will never be more than 40MB/sec/node in this example.