With this commit we rename the existing AvailableDiskSpace function to
its correct name AvailableDiskSpaceRatio as it only returns a ratio. We
then go ahead and add a new function that returns the actual number of
free bytes available on a file system.
This also fixes some comments and always returns an error instead of
panicking.
In this commit, we bump up the logging for failed healthchecks from
`debug` to `info`. This should help us get to the bottom of the current
set of reported false positives that are causing `lnd` nodes to
erroneously shutdown.
Add a new health check package which will periodically poll health
check functions and shutdown if we do not succeed after our set number
of attempts. The first check that we add is one for our chain backend,
to ensure that we are connected to a bitcoin node.
