Revert default setting for # of output workers

[suyograo]

PR #4391 set # of output workers to be equal to # of pipeline workers, by default in 2.2. At that point we weren't fully aware, but it can cause issues like overused resources in outputs. Until every output has been audited and converted to be threadsafe, we should go back to the default of 1 for output workers.

See ref: #4754

[suyograo]

Fixed in #4904