lib: changed functional logic in cluster schedulers

Free pool in round_robin scheduler is implemented as an array. There
were constant lookups being for distributing load on other workers in
free pool. Reimplementing in Map will create will be more performant as
compared to Array implementation. This was done for all in past but free
wasn't implemented at that time.

PR-URL: https://github.com/nodejs/node/pull/32505


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>