Check if we can spend or stake remote staking outputs

[Nizametdinov]

This PR implements IsMine for remote staking and introduces IsStakeableByMe which checks if an output with given scriptPubKey can be used as a stake.

[Nizametdinov]

At first, I introduced ISMINE_STAKEABLE but after merging with ISMINE_HW_DEVICE I ended up with the following enum:

enum isminetype
{
    ISMINE_NO = 0,
    ISMINE_WATCH_ONLY = 1,
    ISMINE_STAKEABLE = 2,
    ISMINE_SPENDABLE = 4,
    ISMINE_FULL = ISMINE_STAKEABLE | ISMINE_SPENDABLE,
    ISMINE_HW_DEVICE = 12, // 0b1100, implies ISMINE_SPENDABLE
    ISMINE_HW_FULL = ISMINE_STAKEABLE | ISMINE_HW_DEVICE, // spendable with hardware wallet and stakeable
    ISMINE_ALL = ISMINE_WATCH_ONLY | ISMINE_FULL
};

I thought that it looked horrible and decided to rewrite it and to introduce a separate function IsStakeableByMe.

[cmihai]

utACK ea295d2.

Note on your worries about the enum: ISMINE_HW_FULL = ISMINE_STAKEABLE | ISMINE_HW_DEVICE is not a real-life usecase, because it would require the user to unlock his hardware wallet every time he wants to propose a block, and we want this to be a periodic, automatic process. So the enum would only need one additional member, ISMINE_STAKEABLE.

The solution with IsStakeableByMe is okay too, though.

[Nizametdinov]

@cmihai ISMINE_HW_FULL = ISMINE_STAKEABLE | ISMINE_HW_DEVICE is possible when some remote staking has a non-hardware staking key and a hardware spending key. In such case, the user will need to unlock her hardware wallet for spending but not for staking.

[scravy]

utACK ea295d2

[scravy]

utACK 22b1e2a

Thanks for addressing these!

[Gnappuraz]

LGTM, utACK 22b1e2a