Add function+tests for exporting a watching wallet.

This change introduces a new function to export a wallet in memory to
a watching wallet.  Watching wallets allow to watch for balance
changes and transactions to wallet addresses while only storing the
public parts of a wallet (no private keys).  New addresses created by
the watching wallet will use pubkey address chaining and will allow to
receive funds to an indefinite number of new addresses, and create the
private keys for said addresses from the non-watching wallet later.

The actual exporting of a watching wallet to a file (triggered by an
RPC request) is not yet implemented.

While here, fix an issue found by new test code for the chained
address code which incorrectly set the starting index of addresses in
the chain needing private keys to be created.