Refactor storage::kv to reduce abstraction duplication

[brson]

This module contains its own traits that define the properties of a key-value store. These traits mostly duplicate engine_traits, while also introducing a few other capabilities, while also leaking details of rocksdb. I'm finding this module to be troublesome as I look for more opportunities to remove rocksdb-specifics from the tikv crate, and think it could use some cleanup.

I propose changing this module to mostly reuse the traits from engine_traits, adding new methods directly to engine_traits where it makes sense, and leaving other methods as extensions traits where those methods don't belong in engine_traits.

Specifically:

• Rename all the *Ext traits in engine_traits to *EngineExt to indicate these are extensions to the main KvEngine trait. This step is needed to make room for new *Ext traits in the next step.
• Rename kv::{Engine, Snapshot, Iterator} to KvEngineExt, SnapshotExt etc.
• Make these new extension traits extend the traits in engine_traits, while deleting the duplicate methods.

There's definitely more cleanup that can be done from there (like putting btree_engine into its own crate), but these steps are about as far ahead as I can see offhand. I think getting this done would go a long way toward simplifying the kvengine abstractions within the tikv crate.

cc #6402

[brson]

Some obvious followups:

• Move btree_engine to its own crate, implementing engine_traits
• Same for mock_engine
• Delete the rocks_engine implementation inside tikv

Any kv engine abstractions within tikv should depend only on engine_traits.

[brson]

It might be easier to start by moving btree_engine and mock_engine into their own crates.

[brson]

This module uses very few other tikv modules, so once it is sufficiently cleaned up and minimized, it may be able to live in its own crate, maybe tikv_kv_store. Right now it doesn't look to me that it will be appropriate to completely merge this module into engine_traits, but it's hard to predict. I think this module can be cleaned up in many small ways.