variable-length column store insert locking in format test program can stall eviction

Summary:
Rework locking in a test program to avoid eviction stalls.

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http://build.wiredtiger.com:8080/job/wiredtiger-test-race-condition-stress-sanitizer/34847/

Variable-length column store uses a global lock for tracking new records inserted into the store, and in runs with high percentages of insert records, that lock can stall eviction.