Allow an arbitrary amount of shared private keys

Currently, because of `Bitcoin<>Monero` pair, one Monero private key is shared among participants, the private view key. This is used to monitor the locked monero.

Depending on pairs, zero or more than one private key may be shared. The specs should defined a list of shared keys (zero or more). Identifiers and meaning of each private shared key is left to the `Arb<>Acc` pair definition.