[Usability] Extract confirmations into connectors

[MaksymZavershynskyi]

Currently EthOnNearClient decides what is the number of confirmations until the header is considered to be confirmed. However, EthOnNearClient does not need to decide, and instead "connector" contract, e.g. MintableFungibleToken can decide for itself. It is actually, preferable, since different applications/tokens might want to have different confirmation thresholds. In case of a generic token connector https://github.com/near/rainbow-token-connector we can do either of the following:

• Different instances of MintableFungibleToken deployed by the factory will have different num of confirmations;
• All instances of MintableFungibleToken deployed by the factory will have the same num of confirmations.

This suggestion will change contract API and therefore it is preferable to do soon.

Assigning to @abacabadabacaba . @abacabadabacaba if you are not comfortable with this task yet, please re-assign to @ailisp

[MaksymZavershynskyi]

Assigning to @mfornet since @abacabadabacaba is working on Solidity part in the next couple of weeks.