Use xpub instead of a single address

[stepansnigirev]

xpub request to arduino return master public key that can be used to derive all addresses.
The derivation path for receiving addresses will be m/0/n, for change addresses - m/1/n

See https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki
m / purpose' / coin_type' / account' will be our xpub returned from Arduino,
then we derive / change / address_index from that.