Fix a long-standing test/format bug that meant values repeated *way* more often than configured.  This showed up because repeating overflow items with large pages result in pathological (quadratic) performance degradation.
Make LSM chunk size configurable in test/format.
Avoid bouncing the session cursor count between zero and one when searching an LSM tree: increment the session's cursor count at the beginnging of an LSM search and decrement at the end.  This also means we can get rid of the special purpose "cache busy" flag.
Use a flag in LSM cursors to track whether the cursor has incremented the session count, clean it up in a few more places in case error paths leave it set.