Consider refactoring so that caesium can be used in isolation from `atlassian-scheduler-core`

Although atlassian-scheduler-api doesn't have any significant baggage to it, atlassian-scheduler-core has requirements like the tenancy API that should really decorate the bare implementations instead of being something that gets injected into them.

Changing this probably mean reworking parts of atlassian-scheduler-core, but that wouldn't necessarily be a bad thing.