Add lookahead to LWS import wallet spec

[vtnerd]

There is one small issue with #647 - it's impossible to do a proper "lookahead" when doing an "import" or "recovery" scan. Basically, the client is forced to do constant /import_wallet_request calls every time it observes a new subaddress in one of the /get... endpoints. This isn't really practical.

Instead, a lookahead and height field is added to the /import_wallet_request endpoint, and the server asynchronously returns failure if the subaddress max is reached in one of the /get... endpoints.

#647 should still be merged as those endpoints are useful for new wallets where no re-scanning is required.

This requires #647 as the address_meta object is defined in that PR but not this one.

I'm also going to update the get_subaddrs endpoint with a new field to return max subaddresses permitted, but lets get #647 merged first.