Perform address indexing in distinct server thread pool.

[evoskuil]

This would decouple server sync with address indexing. As a result the server would be optimized for node queries, which demand minimum latency, while at the same time supporting address (wallet) queries as the node catches up. This would require that the server read each tx from the store in order to index it for the server (history/stealth), so the combined time would increase slightly. This would also allow a node store to be upgraded to a server store. The server height query should be factored to reflect the validated height vs. the indexed height.