Allow arbitrary initial version

[erikgrinaker]

IAVL currently starts from version 1. With non-zero genesis support in Tendermint, applications will need to start from an arbitrary version. Since we already allow both imports at arbitrary versions and deletion of old versions, this is conceptually not a problem.

Suggested API: add Options.InitialHeight, which can be passed to NewMutableTreeWithOpts(). If any versions already exist below InitialHeight, return an error. This is only used during the first SaveVersion() call.