`as-of` should no-op if the passed t value is the same as t on the snapshot

[tylerwilding-wk]

Right now, as-of can do extra work when you, say, have a database at tx 100 and:

(-> db (as-of 50) (as-of 50))

compared to:

(-> db (as-of 100))

The second as-of will still end up reading at least some from storage and do some reads on the indexes to initialize state superfluously.