Fixing channel deletion to include drains

Channel deletion would delete only the ETS table entries for each drain,
leaving the door open for new nodes and NSYNC'ing to bring back dead
drains (that were not killed on existing nodes).

This patch makes it so that deleting a channel, which calls
logplex_drain:delete_by_channel/1 actually removes said drain via redis.
nsync callbacks will then clean up the rest.