Summary:
In D19581 I made `ForwardIterator::status()` check all child iterators,
including immutable ones. It's, however, not necessary to do it every
time -- it'll suffice to check only when they're used and their status
could change.
This diff:
* introduces `immutable_status_` which is updated by `Seek()` and `Next()`
* removes special handling of `kIncomplete` status in those methods
Test Plan:
* `db_test`
* hacked ReadSequential in db_bench.cc to check `status()` in addition to
validity:
```
$ ./db_bench -use_existing_db -benchmarks readseq -disable_auto_compactions \
-use_tailing_iterator # without this patch
Keys: 16 bytes each
Values: 100 bytes each (50 bytes after compression)
Entries: 1000000
[...]
DB path: [/dev/shm/rocksdbtest/dbbench]
readseq : 0.562 micros/op 1778103 ops/sec; 98.4 MB/s
$ ./db_bench -use_existing_db -benchmarks readseq -disable_auto_compactions \
-use_tailing_iterator # with the patch
readseq : 0.433 micros/op 2311363 ops/sec; 127.8 MB/s
```
Reviewers: igor, sdong, ljin
Reviewed By: ljin
Subscribers: dhruba, leveldb, march, lovro
Differential Revision: https://reviews.facebook.net/D24063
35c8c814e8