Implement order cancellation

**Description:** Implement order cancellation and, some refactors to pave the ground for it:

 * Tell apart PeerActor and DefaultPeerActor to avoid cyclic dependencies
 * Make OrderSet handle more edge cases by using a map by price for bids and other for asks

Also, enable the corresponding IT.

**Testing:** new tests added and a new IT pass
**Coverage:** about 85% in both common and client modules
nit: ugly spacing