Implemenet clear/drop on stores

[ncloudioj]

LMDB provides mdb_drop with two flavors:

• Clear all the kv pairs in the given store and keep the store
• Drop the store, which truncates the store and also deletes it in the environment

lmdb-rs has two separate functions for this, txn.clear_db and txn.drop_db, respectively. The latter is marked as unsafe, because the underlying store will be unsafe to use after the call. Rkv can't enforce that, and it's all up to the consumers.

@mykmelez For kvstore, shall we focus on the common case "clear" for now?

[mykmelez]

@mykmelez For kvstore, shall we focus on the common case "clear" for now?

Yes, that seems reasonable to me. We can think more about how to drop stores safely in the future.

[ncloudioj]

Closed via #127