Summary:
I noticed that memtable iterator usually crosses the `iterate_upper_bound`
threshold when tailing. Changes introduced in D43833 made `NeedToSeekImmutable`
always return true in such case, even when `Seek()` only needs to rewind the
memtable iterator. In a test I ran, this caused the "tailing efficiency"
(ratio of calls to `Seek()` that only affect the memtable versus all seeks)
to drop almost to zero.

This diff attempts to fix the regression by using a different flag to indicate
that `current_` is over the limit instead of resetting `valid_` in
`UpdateCurrent()`.

Test Plan: `DBTestTailingIterator.TailingIteratorUpperBound`

Reviewers: sdong, rven

Reviewed By: rven

Subscribers: dhruba, march

Differential Revision: https://reviews.facebook.net/D45909