Threads are re-created everytime we start a new search

[ldufr]

This come with some cost, especially when incremental search are enable.

Unfortunately, it's currently baked in the crate we use to do the directory walk. See here: https://github.com/BurntSushi/ripgrep/blob/fe97c0a152cabc1bc07ec36b4b1e27cd230c3014/crates/ignore/src/walk.rs#L1285-L1304