Configuration for epochs and slots.
Epochs mark a period of time composed of _slots_, for which a particular
[leader schedule][ls] is in effect. The epoch schedule determines the length
of epochs, and the timing of the next leader-schedule selection.
[ls]: https://docs.solanalabs.com/consensus/leader-rotation#leader-schedule-rotation
The epoch schedule does not change during the life of a blockchain,
though the length of an epoch does — during the initial launch of
the chain there is a "warmup" period, where epochs are short, with subsequent
epochs increasing in slots until they last for [`DEFAULT_SLOTS_PER_EPOCH`].
[`DEFAULT_SLOTS_PER_EPOCH`]: https://docs.rs/solana-clock/latest/solana_clock/constant.DEFAULT_SLOTS_PER_EPOCH.html