feat(rest-kv): Add support for bulk operations

[tomxey]

Some requests to the indexer that may require fallback use of the REST KV may need to read data of many transactions/objects at once. (e.g. to compute object and balance changes of a transaction that touches 100 objects, we will need to fetch those 100 objects from the KV, along with the transaction).

To not flood the server with thousands of requests in such cases, we can introduce multi-get requests to the REST KV server, so that requests for multiple keys can be handled in scope of single request.