when throttling on user command there really is no need for injecting heartbeat. The user commanded, therefore gh-ost complies and trusts the reasoning for throttling. What this will allow is complete quiet time. This, in turn, will allow such features as relocating via orchestrator/pseudo-gtid at time of throttling
this optional flag gives --critical-load a second chance. When configured to positive value, meeting with critical-load spawns a timer. When this timer expires a second check for critical-load is made; if still met, gh-ost bails out.
By default the interval is zero, in which case gh-ost bails out immediately on meeting critical load.
