API preconditions should be enforced at the API layer

## Expected Behavior

Preconditions for custom APIs should not be required. For example `Delete` should not fail if there is a write conflict.

## Actual Behavior

Right now preconditions are enforced at the storage layer, so any API that needs to update a resource have preconditions automatically enforced.

## Specifications

We should make preconditions optional at the storage layer, so that custom APIs have the flexibility to skip preconditions if desired.