Fix Bug: iterator.Prev() or iterator.SeekToLast() might return the first element instead of the correct one

Summary:
Recent patch https://reviews.facebook.net/D11865 introduced a regression bug:

DBIter::FindPrevUserEntry(), which is called by DBIter::Prev() (and also implicitly if calling iterator.SeekToLast())  might do issue a seek when having skipped too many entries. If the skipped entry just before the seek() is a delete, the saved key is erased so that it seeks to the front, so Prev() would return the first element.

This patch fixes the bug by not doing seek() in DBIter::FindNextUserEntry() if saved key has been erased.

Test Plan: Add a test DBTest.IterPrevMaxSkip which would fail without the patch and would pass with the change.

Reviewers: dhruba, xjin, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13557