prestwich/home indexing (#768)

* refactor: move db into optics-core, pass db to home instantiation

* feature: prototype chain indexer

* chore: add todo comment

* feature: make chunk_size configurable

* refactor: improve DB through out the codebase

* bug: use to in indexer spawn

* feature: tracing in home indexer

* refactor: indexer settings are strings

* feature: add setting to disable replica #767

* feature: store latest leaf_index in db

* refactor: make fetch_leaf dependent only on DB

* feature: add index config to kovan testnet

* chore: increase polygon chunk size to 9999