Consolidate async_refill call into handle_worker_down helper
Add DEFAULT_START_SIZE macro for consistency

Also remove unused TIMEOUT_PAD
Track start_size in keygen_cache state

This makes config handling a bit easier. Sets us up to log progress
during startup.
Track max workers separately from avail_workers in keygen_cache state

This makes the bookkeeping more robust especially when the server
config is tuned live.
Improve startup logging for chef_keygen_cache

Log progress for pre fill step. Log message when pre-fill is complete
with starting cache count.