Before this commit, it was possible for a request to be sent on the
`chanWatchRequests` channel in `WaitForChannelPolicyUpdate` and then for
the `ticker.C` case to select _before_ the `eventChan` select gets
triggered when the `topologyWatcher` closes the `eventChan` in its call
to `handlePolicyUpdateWatchRequest`. This could lead to a "close of a
closed channel" panic.
To fix this, this commit ensures that we only move on to the next
iteration of the select statement in `WaitForChannelPolicyUpdate` once
the request sent on `chanWatchRequests` has been fully handled.
