In order to give an initial response to incoming subscriptions (without
resorting to retrieving that data from chain again) we now store event
log history in state.
Instead of discarding pending-logs entirely after sending out updates,
we add them to the watchdog's history.
Just like pending-logs, we remove from the head during a rewind (though
not before exhausting the pending-logs).
Kicks the update timer on application start, then sets a new timer
whenever it's awoken. This aims to ensure eth-watcher never stops
looking for updates periodically.
Uses the logic existing in azimuth-tracker to implement a new
eth-watcher, which can look at Ethereum nodes for _any_ events, as
opposed to exclusively a subset of the Azimuth contract's events.
Azimuth-tracker will be reimplemented as a dependent of this in
forthcoming commits.
These were deprecated in favor of azimuth-tracker in #1320.
(Azimuth-tracker, however, isn't a general-purpose Ethereum log watcher
tool. Commits to transform it into a more broadly useful tool are
forthcoming.)
