Replace AFS.verifyReserved with a condition

**Is your feature request related to a problem? Please describe.**
With the changes from https://github.com/apache/accumulo/pull/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 requireReserved();`. verifyReserved() would no longer be needed for UserFateStore, but would still be needed for MetaFateStore.

This should wait until after https://github.com/apache/accumulo/issues/4907 is completed

Start by reading the FateMutator, UserFateStore, and MetaFateStore code around requireReserved() and verifyReserved(), then review the prerequisite issue 4907. Done means UserFateStore uses a reservation condition for operations such as push() and pop(), while MetaFateStore still uses verifyReserved().

Written by the indexing model from the issue text.