Need to block processing tasks until all plugins are loaded

[obourdon]

On a virtualized and shared environment which is quite loaded (several users running heavy jobs) snapd is launched using auto discovery path (-a option) to load 10+ plugins (cpu df disk interface load meminfo processes swap smart heka) and a task which runs every 10 seconds and collect all metrics from the above collectors.

Since the load increased on the system, it seems like the task stops right away in error after startup of snap daemon because at the first iteration of metrics gathering, some plugins are not loaded yet.

[candysmurf]

@obourdon, thanks for logging the issue. We'll investigate solutions.