Add func to get next n active addresses.

This adds a new function, ExtendActiveAddresses, to extend the number
of active addresses, generated by the address chain, by any arbitrary
positive number.  This will be used for recovering wallet addresses
from a wallet backup, as well as getting the next as many addresses as
required to keep a watching wallet in sync with an original wallet
with the private keys, or sync an original wallet with new addresses
generated by the watching wallet.