Use hazard references to prevent child pages being evicted during reconciliation of their parent.

refs #896, #898, #899
Cleanup of the fix, implement review feedback and match Keith's version in 4f182499fd097cdeaede49907299b6f32d30af71.
Merge pull request #900 from wiredtiger/rec-child-hazard

Use hazard references to prevent child pages being evicted during reconciliation of their parent