Replace AFS.verifyReserved with a condition

[kevinrr888]

Is your feature request related to a problem? Please describe.
With the changes from #4524, reservations are now indicated in a column for UserFateStore. For writes like push(), pop(), etc. that require the transaction to be reserved, could instead add a condition to the FateMutator that ensures the FateId is reserved.

Describe the solution you'd like
A new method added to FateMutator that adds the condition that the FateId is reserved: FateMutator<T> requireReserved();. verifyReserved() would no longer be needed for UserFateStore, but would still be needed for MetaFateStore.

This should wait until after #4907 is completed