This commit adds a `MarkChannelClosed` method to the tower client DB.
This function can be called when a channel is closed and it will
check the channel's associated sessions to see if any of them are
"closable". Any closable sessions are added to a new
`cClosableSessionsBkt` bucket so that they can be evaluated in future.
Note that only the logic for this function is added in this commit and
it is not yet called.