Storage Node notary deposit dynamic frequency

[carpawell]

After #770, SN could do notary requests in notary-enabled environmental. It requires some GAS notary deposit to the Notary contract, which is done automatically with constant frequency(in blocks) in #770. That mechanism can be a little smarter.

Options:

1. Add some sort of scheduler that can decide when to deposit depending on balance, current epoch, current deposit's validUntil value, etc.
2. Do nothing since SN's balance is not something really important.
3. ...

[alexvanin]

I think it is valid for Inner Ring as well, since it is doing the same deposits.

[alexvanin]

Instead of the complex scheduling, it is possible to make deposits on demand.

Option 3: check deposit balance before notary request, if the balance is smaller than a threshold, make a deposit.
I suppose getting notary and GAS balance are fast RPC. Deposits can be big enough to do them rarely.