Worker: log main thread memory usage on claim #1101

Each time the worker claims, we should log the % memory used by the main thread.

This doesn't include any usage by the child processes in the engine/runs. It just tells you what's up in the worker.

I think we need this:

const { total_heap_size, used_heap_size}  = v8.getHeapStatistics();
console.log(`memory usage: ${used_heap_size} / ${total_heap_size} (%)`)

Later, we can decide what to do if memory is low. Should we alert? Should we force a restart?

We might consider polling this.