CompactionFilterV2: eliminate an often unnecessary allocation.

If a compaction filter implementation is simply filtering values, then
allocating the "changed values" bitmap is an extra memory allocation
that adds no value. Additionally, the compaction implementation has to
do marginally more work to calculate the offset into the bitmap
(vector<bool> specialization) for each record the filter did not mark
for deletion.

Explicitly handle the case where compact_->value_changed_buf_ is empty.
Iterator support for Write Batches
Test for WriteBatchHandler
Addresed comments from code review https://reviews.facebook.net/D27567
Merge pull request #384 from msb-at-yahoo/compaction-filter-2-empty-changed-values

CompactionFilterV2: eliminate an often unnecessary allocation.
Fix -Wnon-virtual-dtor errors

Summary: This breaks mongo+rocks build https://mci.10gen.com/task_log_raw/rocksdb_ubuntu1404_rocksdb_c6e8e3d868660dc66b3bbd438cdc135df6356c5a_14_11_10_21_36_10_compile_ubuntu1404_rocksdb/0?type=T

Test Plan: m check + -Wnon-virtual-dtor

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28653
Move -Wnon-virtual-dtor to c++ flags
Make ForwardIterator::status() more efficient

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
Merge pull request #363 from adamretter/write_batch-iterate

Iterator support for Write Batches