[Tech] Load limiting and congestion control sims

[Michael Rogers]

A quick progress update:

Congestion control is done but not documented.

Load limiting is nearly done: the simulator now includes a simplified 
version of the current backoff mechanism and an incomplete version of 
the new token-passing mechanism. Both can be disabled easily, so I 
should be able to make some preliminary comparisons between the two 
mechanisms (and the third possibility, no flow control) in the next few 
days.

To do (short term):

* A queue for locally generated searches
* Request throttling for the current backoff mechanism - locally 
generated searches should be released from the queue at a rate that 
depends on the number of RejectedOverloads they cause
* Fullest-bucket, emptiest-bucket and random-bucket policies for the new 
token-passing mechanism
* Debug both mechanisms in small-scale simulations
* Badly-behaved nodes that don't throttle their searches
* Wiki page describing congestion control

To do (long term):

* Heterogeneous nodes
* Swapping
* Churn
* Other social network models

Cheers,
Michael