Use a separate thread pool for checks.

Refs #5050
Include thread pool ID in debug messages.

Refs #5050
Create new threads more aggressively right after program start.

Refs #5050
Use normal check interval for services which haven't been checked yet.

Refs #5050
Don't kill too many worker threads at once.

Refs #5050
Set NPROC limit in the init script.

Refs #5050
Fix latency calculation.

Refs #5050
Merge branch 'feature/threadpool-bug-5050' into next

Fixes #5050