1. Change config format to support multiple types
of snapshotting modes.
2. Implement a hacky way to support periodic or completely
manual snaphots.
In manual mode, the user has to trigger replication using the wakeup
mechanism after they took snapshots using their own tooling.
As indicated by the comment, a more general solution would be desirable,
but we want to get the release out and 'manual' mode is a feature that
some people requested...
Just because taking one snapshot fails does not mean snapper needs to
stop for all others.
Since users are advised to monitor error logs, snapshot-taking errors
can still be addressed.
The ErrorWait mode allows a potential future Report / Status command to
distinguish normal waits from error waits.
