[thread_manager] More asserts.
[thread_manager] More asserts.

victorisildur/Graphite,mit-carbon/Graphite-Cycle-Level,fhijaz/Graphite,mit-carbon/Graphite,victorisildur/Graphite,mit-carbon/Graphite-Cycle-Level,8l/Graphite,mit-carbon/Graphite,mit-carbon/Graphite,8l/Graphite,fhijaz/Graphite,mit-carbon/Graphite-Cycle-Level,8l/Graphite,nkawahara/Graphite,fhijaz/Graphite,8l/Graphite,nkawahara/Graphite,nkawahara/Graphite,fhijaz/Graphite,victorisildur/Graphite,victorisildur/Graphite,mit-carbon/Graphite,nkawahara/Graphite,mit-carbon/Graphite-Cycle-Level