Implemented support for contract renegotiation

Implemented support for contract renegotiation

This patch changes significantly the interface between FCB and resource
managers and allocators. There no longer exist cancel_* method. The main
negotiation methods (manager.reserve_contracts, allocator.change_vreses) now
use an interface which supports the semantic offered by
frsh_group_change_mode_sync() i.e. in one negotiation operation, there might
be a combination of multiple requests to negotiate, renegotiate or cancel
contracts. Resource managers process these requests atomically, i.e.  either
all requests succeed or none of them.

The interface for implementing individual resource managers and allocators
remains the same with one exception - fres_sa_scenario_for_each_contract()
was replaced by fres_sa_scenario_for_each_no_cancel_contract().