ForwardIterator: reset incomplete iterators on Seek()

Summary:
When reading from kBlockCacheTier, ForwardIterator's internal child iterators
may end up in the incomplete state (read was unable to complete without doing
disk I/O). `ForwardIterator::status()` will correctly report that; however, the
iterator may be stuck in that state until all sub-iterators are rebuilt:

  * `NeedToSeekImmutable()` may return false even if some sub-iterators are
    incomplete
  * one of the child iterators may be an empty iterator without any state other
    that the kIncomplete status (created using `NewErrorIterator()`); seeking on
    any such iterator has no effect -- we need to construct it again

Akin to rebuilding iterators after a superversion bump, this diff makes forward
iterator reset all incomplete child iterators when `Seek()` or `Next()` are
called.

Test Plan: TEST_TMPDIR=/dev/shm/rocksdbtest ROCKSDB_TESTS=TailingIterator ./db_test

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: lovro, march, leveldb

Differential Revision: https://reviews.facebook.net/D22575