[circle] caching channels subscribers

[thomasV1]

I just had an idea:

Currently, if you are posting to a channel, circle firsts queries the dht 
for the list of subscribers, and then it sends messages to them.

It would make sense to cache the list of people that are listening to a 
channel. So, when I post to #welcome, my client would:

  - immediately send the message to all #welcome 
    subscribers that I already know (the list in my cache)
  - query the dht in order to know if my list of subscribers was correct
  - send the message to the people that were missing in my cache.
  - update my cache.

if a client receives a message that was posted to a channel to which 
he is not listening anymore, then he would simply not display the 
message, and maybe notify the sender.

It should be faster that way, but I am not sure how much.
Anyway, the cost in terms of messages sent over the network
is almost identical to the non-caching algorithm, so it cannot hurt 
to implement it.

Thomas

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