ForwardIterator::status() checks all child iterators

Summary:
Forward iterator only checked `status_` and `mutable_iter_->status()`, which is
not sufficient. For example, when reading exclusively from cache
(kBlockCacheTier), `mutable_iter_->status()` may return kOk (e.g. there's
nothing in the memtable), but one of immutable iterators could be in
kIncomplete. In this case, `ForwardIterator::status()` ought to return that
status instead of kOk.

This diff changes `status()` to also check `imm_iters_`, `l0_iters_`, and
`level_iters_`.

Test Plan:
  ROCKSDB_TESTS=TailingIteratorIncomplete ./db_test

Reviewers: ljin, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19581