impl ZcashIndexer for StateService

[AloeareV]

This is in need of testing, which I plan to start on today.
Completes #111 and #113

[idky137]

Looks really good!

The only things i can see without more testing is that the impl StateService on line 770 is now mostly defunct. I think it is only the get_block_header method that is now used.

Maybe this should be grouped in an impl StateService with the get_compact_block and to_tx methods from the impl block starting on line 1222.

The get_block_range could be left in its own impl that will be converted to the LightWalletIndexer trait in Milestone 3?

The other thing is that mempool data is also available by adding a zaino-state::Mempool to StateService. This would require the StateService to also implement a StateServiceSubscriber in the same way FetchService does so it may be best to wait until we implement the LightWalletIndexer trait in Milestone 3 for this work.