Fix another bug regarding when pages can be evicted: have reconcilition track the maximum transaction ID on the page (regardless of whether it is skipped).  A page can only be evicted when it is clean and has no updates that are too new.

refs #884
Rename disk_txn to reflect what we're actually using it for.
Add comments explaining the reconciliation transactional read logic, including when pages become clean.
Merge pull request #888 from wiredtiger/rec-skip-clean

Fix a bug regarding when clean pages can be evicted