Simplify handling of previous values

There are 5 ways the user can influence whether the previous value is read from a store/remote node:

• unsafe.unreliableReturnValues
• IGNORE_RETURN_VALUES
• SKIP_CACHE_LOAD
• SKIP_REMOTE_LOOKUP
• CACHE_MODE_LOCAL

Currently the effect of each is slightly different, depending on whether the operation is conditional, whether it's a delta-aware write etc. This is how I think it should work:

• unsafe.unreliableReturnValues and IGNORE_RETURN_VALUES should skip reading the previous value only if it doesn't change the outcome of the operation (i.e. no effect for reads, conditional operations, or delta-aware).
• SKIP_CACHE_LOAD should always skip the cache loader.
• SKIP_REMOTE_LOOKUP and CACHE_MODE_LOCAL should always skip the remote lookup