HTTP endpoint: get next nonce

[velzevur]

Sending transactions for the same account from a distributed system requires knowledge for the latest account nonce. Since transactions can be included, rejected or even replaced by transactions with a competing nonce, the only source of truth shall be either a wallet or a synced node.

Provide an endpoint that returns what the next nonce for an account would be, taken into consideration transactions in the pool, originating from the same account.

[marc0olo]

I always thought about a tx-manager on client side that handles nonce-management (which of course would still make sense). but this is definitely an interesting solution.