Added a read rate limiter which allows limiting the read QpS per client

[kelindar]

This can be used in order to protect the broker from misbehaving clients. This PR also changes the previous writeRate to flushRate which is a more appropriate name.

	// The maximum messages per second allowed to be processed per client connection. This
	// effectively restricts the QpS for an individual connection.
	ReadRate int `json:"readRate,omitempty"`

	// The maximum socket write rate per connection. This does not limit QpS but instead
	// can be used to scale throughput. Defaults to 60.
	FlushRate int `json:"flushRate,omitempty"`