fix: manager started race HEAD trunk

It's possible for there to be a data race where `Add` checks if the
manager has started, then proceeds to append to the runnables slice, at
the same time that `Start` is progressing towards starting the manager.
This results in a data race with the runnables slice.

This changeset modifies the manager to use a more traditional locking approach.

0 files changed, 0 insertions, 0 deletions