Consider adding option to disable updates to access time AVU

[korydraughn]

Today the storage tiering plugin assumes an access time AVU is always attached to a data object. But should it?

Consider the case where access time is never used in the violating query.

With the current implementation, even if the violating query doesn't rely on the access time, the access time is still required to exist on each data object. If the access time is not attached to the data objects being tiered, the plugin skips that data object and returns CAT_NO_ROWS_FOUND.

Related to #72 (comment).

[alanking]

See #72 (comment) for pitfall should we implement this.