Increase default metadata_workers, backlog to 4096

While deployment tools might want to change the number of workers
to match the number of cores (Or some fraction of it), any default
other than 1 should be outright better.

Inspired from 19:34:
https://www.youtube.com/watch?v=AF9r_VQrcJ0

DocImpact
Closes-Bug: #1323005
Change-Id: Ie90000183ae67ff391a23ca3213fd23aef5f4dc5
Merge "Increase default metadata_workers, backlog to 4096"