Standard safe zone

We record

• Number of slots from the tip of the ledger. This should be (at least) the number of slots in which we are guaranteed to have k blocks.
• Optionally, an EpochNo before which no hard fork can take place.

Pretend the transition to the next era will not take place.

This constructor is marked as unsafe because it effectively extends the safe zone of this era indefinitely into the future. This means that we might reach invalid conclusions when doing

• slot to time conversions for blocks that are past the actual safe zone
• time to slot conversions for the current time, when behind in syncing

This is safe when the code is simply not yet ready to transition to the next era, because in that case, we can be sure that blocks that come in are still from this era. It also means that we can always produce a block, no matter how far ahead of the current ledger we are.

If the code is ready for the transition, just awaiting an update proposal, then LowerBound can be used instead.

This constructor can be regarded as an " extreme " version of LowerBound, and can be used for similar reasons.