Similarly as with kvdb.View this commits adds a reset closure to the
kvdb.Update call in order to be able to reset external state if the
underlying db backend needs to retry the transaction.
This commit moves makeTestDB to db.go and exports it so that we'll be
able to use this function in other unit tests to make them testable with
etcd if needed.
Add a new top level bucket which holds closed channels nested by chain
hash which contains additional information about channel closes. We add
resolver resolutions under their own key so that we can extend the
bucket with additional information if required.
