message: Making MergeCandidate public in order to allow the usage of custom merge
  policies.
  Closes
diff: @@ -237,6 +237,7 @@ pub fn version_string() -> &'static str {
  pub mod merge_policy {
  pub use crate::indexer::DefaultMergePolicy;
  pub use crate::indexer::LogMergePolicy;
  + pub use crate::indexer::MergeCandidate;
  pub use crate::indexer::MergePolicy;
  pub use crate::indexer::NoMergePolicy;
  }
  