enable stake table epochs

[alysiahuggins]

Closes #2505

This PR:

• re-adds epoch functionality so that validator registrations and exits can be handled on a epoch basis
• ensures that the currentEpoch is a based on the latest hotshot block height and the hotShotBlocksPerEpoch
• the hotShotBlocksPerEpoch is set at constructor time
• consolidates nextRegistraionEpoch() and nextExitEpoch() into one function, nextAvailableEpoch(...) since the logic was the same
• adds tests for the currentEpoch() and nextAvailableEpoch(...) functions

This PR does not:

• provide a way to update hotshot blocks per epoch as it's assumed that this will not change in the near future, when it does change the function can be updated
• handle integer overflows (e.g. currentEpoch()+1) - this is a discussion to have
• modify epoch logic for functions such as withdrawFunds() which depends on the epoch functionality, this will be handled in other github issues.

Key places to review: