Groundwork for keepalives for all services

[jinnatar]

This PR deprecates enabling or disabling individual handlers in favor of service level enablement, which in turn enables all handlers defined for that service.

More ground work for fixing #361. At this PR it's already possible to disable harvester monitoring but there are no other keepalives available yet. The next PR in the chain adds a new keepalive for the wallet as an example.

I'm torn on which services should be monitored by default. The conservative default would be no services, but that clashes with the previous default of running all handlers and the harvester keepalive. So I've opted to default to monitoring all services and thus all handlers, which also enables the harvester keepalive. Where it gets a bit wonky then is when we bring in a wallet keepalive. There's been no harm in having the incoming transaction handler enabled by default, but by comparison the peak handler expects to find a working wallet. If one isn't found, it will start triggering keepalive alerts of no wallet activity.

[jinnatar]

The sneak peek of the wallet peak handler and wallet keepalive: https://github.com/Artanicus/chiadog/compare/keepalive...Artanicus:chiadog:keepalive_wallet?expand=1

[jinnatar]

Current code coverage overall with most notifiers enabled (8 tests skipped and Grafana failing) is 78%. Not bad, not great. Keep-alive is at 87%. Biggest misses are:

• Remote keep-alive isn't tested. I've previously used mbtest for these kinds of things but it's a really heavy testing dependency so will have a go at using VCR.py since it's way more self contained and faster.
• We don't test what happens without a NotifyManager, probably should.
• A couple of error conditions are not triggered, probably better to just exclude those from coverage.

Then there's actually testing novel situations, will have a think of those wile getting the pure coverage up first.