box: introduce memtx_sort_threads config parameter

Closes #3389
Closes #7689
Closes #4646

@TarantoolBot document
Title: new box.cfg parameter memtx_sort_threads

The parameter sets the number of threads used to sort keys of secondary
indexes on loading memtx database. The parameter cannot be changed
dynamically (as it does not make sense).

Maximum value is 256, minimum is 1. Default is to use all available cores.

Usage example:
```
box.cfg{memtx_sort_threads=4}
```