This also means that modules do *not* have to call bar->refresh() just
before/after calling module_signal_ready().
As long as the module's initial state has been correctly/completely
set up before it calls module_signal_ready(), the module *only* has to
call bar->refresh() when it's state changes.
After starting all the module threads, wait for all modules to have
signalled "ready" before continuing.
This will allow modules to do initial setup, and knowing that
content() will *not* be called until they've signalled "ready".
