For those operating in an async environment, requiring
ChainAccess::get_utxo return information about the requested UTXO
synchronously is incredibly painful. Requesting information about a
random UTXO is likely to go over the network, and likely to be a
rather slow request.

Thus, here, we change the return type of get_utxo to have both a
synchronous and asynchronous form. The asynchronous form requires
the user construct a AccessFuture which they clone and pass
back to us. Internally, an AccessFuture has an Arc to the
channel_announcement message which we need to process. When the
user completes their lookup, they call resolve on their
AccessFuture which we pull the channel_announcement from and
then apply to the network graph.

Fixes #1975.