Investigate increased CPU load for extra peers

[shane-kerr]

It seems like adding extra peers increases CPU load. A small number of peers (like 10 or so) has the wallet idle most of the time, whereas a large number of peers (like 100 or so) has the wallet using tons of CPU.

We should investigate this, as sending data to a peer should be relatively cheap. Perhaps we are checking all of the blockchain sent from all peers, which is a waste of time (both CPU time and wall clock time)?