Allow plugins.checkFrequency = 0 to disable scanner

Production systems might not want to scan for plugins, and instead
rely on system administrators to invoke `gerrit plugin reload` or
`gerrit plugin install` to make explicit plugin changes. Disable the
background scanning thread when the check frequency is set to 0 or any
negative value.

Change-Id: Ib0c1b40db51c960650099e5ae84bda6aebbc572e