close #743

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

  • Previously we always transformed between KeyRange and Interval<BytesAffine>, transform through &[u8]. This may make unnecessary Vec clone and memory allocation.
  • And the conflict check in KeyRange is too confusing and troubling due to Bounds.

• what changes does this pull request make?

  • now KeyRange is refactored:

    pub enum KeyRange {
        OneKey(Vec<u8>),
        Range(Interval<BytesAffine>),
    }

    and it can convert between EtcdKeyRange and Interval<BytesAffine> without cloning.
  • The conflict check is also convenient by using Interval::overlap.
  • The reason spliting KeyRange to OneKey and Range is to distinguish prefix and onekey, which may has same form in [a, a+1).
  • refactored some KeyRange usage in crates/xline/src/storage/index.rs and crates/xline/src/storage/kv_store.rs. By using KeyRange directly instead of start: &[u8], end: &[u8] maybe better to construct from one key or all keys, and reduce the clone times.

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)