Recheck if empty under lock, early exit

[benaadams]

Changes

• Once the ConcurrentDIctionary lock is acquired, recheck if the collection is empty before trying to clear it
• Remove some additional clears of preWarm cache as over clearing them
• Allow prewarm cache to clear in background on prewarm continuation; only forcing at start of next block if not cleared

Types of changes

What types of changes does your code introduce?

• Optimization

Testing

Requires testing

• No