wallet: change _requests_addr_to_key map to multi-map
wallet: change _requests_addr_to_key map to multi-map

I find this easier to reason about than occasionally overwriting the items.
get_request_by_addr still only returns a single invoice for simplicity,
but now all logic regarding how to handle collisions is inside that method.

spesmilo/electrum,spesmilo/electrum,spesmilo/electrum,spesmilo/electrum