Running small but cpu intensive tasks in a thread pool.

This PR removes usage of spawn_blocking for cpu intensive tasks.

The problem with spawn_blocking is that these tasks get scheduled on a
ever growing thread pool.

For instance, when the server is under load, the GZIP decompression of
payloads could considerably increase the load factor of quickwit,
possibly making it unresponsive to healthcheck.

This PR isolates the thread pool used for the searcher, and instantiates
a second generic thread pool dedicated to those short cpu-intensive tasks.