Allow Python tests to detect whether verbose support is configured.
Add a "verbose=[transaction]" mode, output a message if a snapshot falls a long way behind.  Add a test case that generates this message.  Remember the last operation on a session after it leaves the library.
Copy values into positioned cursors in transaction commit so that the oldest ID doesn't need to stay pinned.
Don't try to force eviction on a page if the oldest transaction ID hasn't moved on since last time we tried.
Fix an unused variable warning for non-diagnostic builds.
Fix the Python check for a verbose build: it can't be a connection method, that's too late.
Merge pull request #1291 from wiredtiger/snap-min

Improvements for long-running transactions