Add critical health check for number of workers on cooldown

[kwypchlo]

• added new critical health check that fails when number of workers on cooldown to total number of workers ratio is more than a certain threshold (currently set to 60%)
• dropped passing external server ip on local skyd check (would falsely report error every time, harmless since it was just printing that when real issue occurred but pretty confusing)
• critical fix: registry entry debug serialiser was failing since entries have BigInt revision (whole health check fails to finish if that happens, very critical bug fix)

[linear]

SKY-1112 Add critical health check for number of workers on cooldown