Improve Handling of Job Command Plugin Database Connections

Job Command forks child processes in parallel, which causes problems if a child process exits (even a normal exit) while sharing the same database connection as the parent. To work around this, JobCommand creates a plugin database connection for use by the child process.

It should be possible for JobCommand to do something like
ConnectionManager::alias('plugin', 'default');
so that the connection on each table doesn't need to be reset, but this doesn't actually seem to work.