In this commit, a new method `cleanClosedChannels` is added and called
when a circuit map is created. This method will delete the payment
circuits and keystones for closed channels.
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.
Previously, we would only allow deletion of circuits if all circuit keys
were found in the pending map.
In this commit, we relax this to allow for deletion of any circuits
that are found pending, and ignore those that are not found. This
is a preliminary step to cleaning up duplicate forwards that get caught
by the switch. It also allows us to gracefully handle any nodes that
are still afflicted by the split mailbox issue.
Replaces the log statement in CommitCircuits so that
it prints the circuit key of the incoming channel. This way
we avoid spewing the secp curve stored in the ErrorEncrypter.
