Add option for hashing_threads, use 10 asyc IO threads
Add option for setting lt::settings_pack::hashing_threads
introduced by libtorrent 2.0. It has no effect for earlier
libtorrent versions.
Use 10 async IO threads, as per #11461.
Closes#11461.
<labelfor="filePoolSize">QBT_TR(File pool size:)QBT_TR[CONTEXT=OptionsDialog] <ahref="https://www.libtorrent.org/reference-Settings.html#file_pool_size"target="_blank">(?)</a></label>