Revert "Revert "go-url-shortener: set GOMAXPROCS""

This reverts commit c6deec5e3137889990b942c595e20b22cc162995.
Revert "Revert "init.go: rely on GOMAXPROCS being set from outside (via env variable)""

This reverts commit 0cbf0aa78b5de7d306c821587abeb80a25d58550.