Adding basic monitoring of drains & channels

Looks for orphans (both in ETS vs. drain, or channels vs. drains, in any
storage available).

Also looks at total drain and channel count, and compares it with other
nodes.
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.

Conflicts:
	src/logplex_drain.erl
	src/logplex_mon_serv.erl
Minor fixes
Increasing kill time on health monitor

The health monitor's own supervisor gives 8 seconds and it's useless to
give only 2 to its parent. The 8 seconds might be required because
scanning entire tables for orphans might be a bit long before seeing
sys messages.
Upgrade path v65 -> v66
Reducing delay to 5 minutes for leak monitor

The operations are long-lived and there isn't a lot of expected benefits
to having things run very frequently. A 5 minutes check ought to be
sufficient.
Removing lengthy redis checks

Looking for drains is too expensive as a regular operation. In
production it may take up to two minutes just to run all the checks and
Redis is what makes it slow. Tests have shown that no drain currently on
a production node appears to be orphaned in either ETS or Redis, but
always both. As such, we drop this lookup.