PlainTableIterator::Seek() shouldn't check bloom filter in total order mode

Summary:
In total order mode, iterator's seek() shouldn't check total order.

Also some cleaning up about checking null for shared pointers. I don't know the behavior before it.

This bug was reported by @igor.

Test Plan: test plain_table_db_test

Reviewers: ljin, haobo, igor

Reviewed By: igor

CC: yhchiang, dhruba, igor, leveldb

Differential Revision: https://reviews.facebook.net/D17391