Consider providing compaction support in the Backend interface

I'm implementing my own backend, and may want to add compaction support sometime in the future. IMO this needs to be an opt-in feature, so when a user chooses to compaction will take place. I'm also pretty sure a background thread running all the time doing this is not the way to go.

Can OdbBackend have a DoCompaction method that if implemented will allow it to perform compaction on demand? (or similar)

This will also probably require libgit2sharp to implement and expose the various helpers for performing binary diffs and object concats.

Related / trigger: https://github.com/synhershko/libgit2sharp.Elasticsearch/issues/2

/cc @nulltoken @carlosmn

先檢視 OdbBackend 介面以及相關的 libgit2sharp.Elasticsearch issue，以了解提議的 opt-in 壓縮 API。檢查二進位差異和物件串接所需的 libgit2sharp 支援；完成的標準是就介面設計達成共識，並在不進行背景壓縮的情況下公開必要的 helper。