Commit 40d964ec allowed vacuum command to leverage multiple CPUs by
invoking parallel workers to process indexes.  This commit provides a
'--parallel' option to specify the parallel degree used by vacuum command.

Author: Masahiko Sawada, with few modifications by me
Reviewed-by: Mahendra Singh and Amit Kapila
Discussion: https://postgr.es/m/CAD21AoDTPMgzSkV4E3SFo1CH_x50bf5PqZFQf4jmqjk-C03BWg@mail.gmail.com