message: Use BTreeMap instead of OrdMap in merkle storage
  Remove external dependency and use standard BTreeMap. It seems OrdMap was
  contributing to high memory use.
  Once we stabilize memory usage we can look at replacing it with immutable
  version if it has better performance.
diff: @@ -46,10 +46,9 @@ use std::path::Path;
  use rocksdb::{DB, ColumnFamilyDescriptor, Options, IteratorMode};
  use crate::persistent::{default_table_options, KeyValueSchema};
  use std::hash::Hash;
  -use im::OrdMap as OrdMap;
  use serde::Deserialize;
  use serde::Serialize;
  -use std::collections::HashMap;
  +use std::collections::{HashMap, BTreeMap};
  use failure::Fail;
  use std::sync::Arc;
  @@ -71,7 +70,7 @@ struct Node {
  entry_hash: EntryHash,
  }
  -type Tree = OrdMap<String, Node>;
  +type Tree = BTreeMap<String, Node>;
  #[derive(Debug, Hash, Clone, Serialize, Deserialize)]
  struct Commit {
  