Remove Memory storage engine.

With BitCask now the main storage engine, there's no point keeping a
custom in-memory B-tree implementation around. Remove it in favor of the
stdlib implementation.

config/toydb.yaml

@@ -26,6 +26,5 @@ storage_raft: hybrid
 
 # SQL key-value storage engine
 # - bitcask (default): uses BitCask, an append-only log-structure store.
-# - memory: uses an in-memory B+tree. Durability is provided by the Raft log.
 # - stdmemory: uses the Rust standard library BTreeMap.
 storage_sql: bitcask

src/bin/toydb.rs

@@ -42,7 +42,6 @@ async fn main() -> Result<()> {
         "bitcask" | "" => {
             Box::new(storage::kv::BitCask::new_compact(path.join("state"), cfg.compact_threshold)?)
         }
-        "memory" => Box::new(storage::kv::Memory::new()),
         "stdmemory" => Box::new(storage::kv::StdMemory::new()),
         name => return Err(Error::Config(format!("Unknown SQL storage engine {}", name))),
     };