[WIP - DO NOT MERGE] Implement ldb database functionality for optional addr index. 

This pull request alters the `btcdb.db` interface, adding new functions to for accessing/updating/deleting the optional address index. For now, only the `ldb` driver implements the newly defined functions. `memdb` has also been altered, but only to supply skeleton functions in order for the package to conform to the new `btcdb` interface. Additionally this PR makes progress towards completion of conformal/btcd#190. Subsequent PR's dependant upon this one will be opened across: `btcd`, `btcjson`, and `btcwire` each moving forward, ultimately fully implementing `btcd`'s new optional `addrindex`. 


[description to be updated after WIP tag is removed]...