When users submit different number of inputs in one mixing session it can potentially leak info by previous txid even if all inputs use the same denomination(s). This is mostly applicable to first few rounds of mixing, the more rounds of mixing is passed the less info is leaked. But anyway, this is still a really bad situation imo.

This PR should fix issue I just described by requiring users to submit not only the same denominations but also exactly the same number of inputs.

Note:

• replaces few value checks (which doesn't make sense together with this PR anymore imo) by "number of inputs" checks.
• bumps PROTOCOL_VERSION and MIN_POOL_PEER_PROTO_VERSION