export: apply delta encoding for better compression rate

[yihuang]

Snapshot is always created and processed in order, taking advantage of that, we can apply delta encodings to some fields:

• version
  IAVL tree has the nice property that parent node's version is always bigger than or equal to children's version, and in snapshot, nodes are added in post-order, where parent node is put after children nodes, so we can only record the delta value, to be specific parent version - max(left version, right version), smaller value results in smaller encoded size with varint encoding.
• key
  After the export: don't export branch node's key in snapshot #688 improvement, we only have leaf keys in snapshot, they are ordered, so delta encoding can also potentially have some good compression rate there.
• height
  since IAVL tree has the invariant that parent height = max(left height, right height) + 1, I think we don't need to encode the height field at all in the snapshot.

The version and height compression rely on the fact that nodes are exported in post-order. the key compression only need the leaf nodes are exported in order.

[yihuang]

Some thoughts about snapshot format migration:

• Make the IAVL library always support all the legacy versions, so an introduction of a new format can be considered as a non-breaking change, let cosmos-sdk handle the versioning of the snapshot in application.
• change the import/export APIs to be format aware, and dealing with raw bytes directly, let IAVL library handle the serialization of ExportNode, it'll be more convenient to introduce new formats.
  • it's a API breaking change, but not behavior breaking.
• add a new api to return the list of supported formats in current version, SDK can check the snapshot format against it, sdk should add a new config option for user to specify which format is used to export a snapshot.