Add support for custom Executor

Currently all the blocking API methods run on the calling thread, and the async methods run on the fork-join pool. We should allow for passing in (or building the client) with a custom `Executor` for async operations.

Start by tracing the client construction and the blocking and asynchronous API methods described in the issue. Define the executor configuration and verify that asynchronous operations use the custom Executor while blocking methods remain on the calling thread; add tests for both behaviors.

Scritto dal modello di indicizzazione a partire dal testo della issue.