Removed silly "const" in pass-by-value paramteer to inform_memory_limit_change.
Removed unused "friend class txn_t".
Gave alt_txn_throttler_t a minimum_throttling_limit_ field.
Made PageTest pass a minimimum unwritten changes limit that is not 1.
Merge branch 'v1.12.x' into sam_2187
Removed the page_test_helper_t wrapping.