Provide our own C interface to rocksdb.

[petermattis]

Stop using the rocksdb C interface and instead provide our own C
interface to rocksdb. This makes it cleaner to implement various bits of
cockroach-specific functionality in C++ such as the compaction filter
logic and merge operator.

[bdarnell]

LGTM

[sourcegraphbot]

 (*engine.RocksDB).CreateSnapshot(snapshotID string) error

 (*engine.RocksDB).ReleaseSnapshot(snapshotID string) error

 (*engine.RocksDB).WriteBatch(cmds []interface{}) error

 func engine.TestEnginePutGetDelete(t *testing.T)

 (*engine.RocksDB).ApproximateSize(start engine.Key, end engine.Key) (uint64, error)

 type engine.RocksDB struct

 func engine.TestRocksDBCompaction(t *testing.T)

 func engine.NewRocksDB(attrs invalid type, dir string) *engine.RocksDB

 (*engine.RocksDB).Destroy() error

 (*engine.RocksDB).Get(key engine.Key) ([]byte, error)

 (*engine.RocksDB).IterateSnapshot(start engine.Key, end engine.Key, snapshotID string, f func(invalid type) (bool, error)) error

 (*engine.RocksDB).Start() error

 (*engine.RocksDB).Stop()

 (*engine.RocksDB).CompactRange(start engine.Key, end engine.Key)

 (*engine.RocksDB).Flush() error

 (*engine.RocksDB).GetSnapshot(key engine.Key, snapshotID string) ([]byte, error)

 (*engine.RocksDB).Iterate(start engine.Key, end engine.Key, f func(invalid type) (bool, error)) error

 (*engine.RocksDB).Merge(key engine.Key, value []byte) error

 (*engine.RocksDB).Put(key engine.Key, value []byte) error

 (*engine.RocksDB).Clear(key engine.Key) error

---

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[spencerkimball]

I've started to regret making the DBBatch a different sort of object. I think we should revert to the Viewfinder model, where a batch is just another type of engine (as is snapshot) and give DBEngine a Flush() method which is normally a noop but executes a write batch in the event the engine is really a DBBatch.

[petermattis]

Heh, guess I didn't beat that horse long enough. Yeah, happy to switch over to the viewfinder model in a subsequent change.

Do you think DBSnapshot should be a type of DBEngine as well?

[spencerkimball]

Yes. Same as with Viewfinder. It'll simplify the go code as well--so annoying to have to call SnapshotGet(), etc. And having to create a batch object and then commit it when you don't really need the batch functionality is a pain.

[spencerkimball]

LGTM