replace ChainStore with an AddOnlyStore

[rkuhn]

There are fewer places in Amaru that update the persistent storage than there are places reading it. Since almost all persistent storage is add-only (with the notable exception of remembering the current best branch tip), it should be quite simple to implement a snapshot mechanism. This allows access to only serialize between snapshot retrievals and additions, with subsequent snapshot reads not needing any synchronization.

I have not yet researched RocksDB to see how well this approach is supported, but if it isn’t supported then we can use some other suitable DB.