[WIP] replace private key identity by mnemonic

[louisinger]

This PR aims to replace all the Private key identity stuffs by the Mnemonic identity

TL;DR

• The walletAction initializes the wallet using a mnemonic seed (or generate a random one).
• The walletBalanceAction fetches the balances of all the addresses derivated from the mnemonic identity.
• The walletSendAction uses all the mnemonic addresses to fetch utxos and use a new change address in buildTx.
• The state stores now the Identity object instead of a single address.

Why store the identity instead of addresses ? We need the identity object to avoid restoration from the explorer each time we need to recreate the Identity object (when we sign the tx for example).

Why it is a problem ? The Identity object contains sensitive data and is not encrypted! Everybody can use it to sign the transactions even if the keystore type is encrypted.

What are the solutions ? first solution: encrypt all the state instead of just the private key. second solution: store a set of addresses instead of the identity itself and overwrite the IdentityRestorer to restore addresses from the state each time we recreate the Identity.

@tiero I need your opinion about that

it closes #1

[tiero]

As per discussed offline, I think we should encrypt the whole Identity instance and whenever we need to interact with the wallet (either reading the addresses or signing) we will ask the password to decrypt.

[tiero]

In the end I think a custom restorer would be the cleanest solution, for initializing from a set of cached addresses @louisinger