This commit creates a new package as sub-package within the routing
package: chainview. This package is centered around a single interface
definition: the FilteredChainView. This interface is to be used to
allow the routing package to watch a _subset_ of the UTXO set for any
modifications. In the case of LN, the subset of the UTXO set that we
care about is the set of currently opened channels.
In a future commit the routing package will be modified to remove the
current full block scanning with processing of FilteredBlock
notification, and proper updates to the filter as observed by the
FilteredChainView.
