Add a section to docs for async operations. #1139
Merge branch 'develop' into async-doc
Review/edit the new asynchronous doc page, lots of random
cleanup/simplifications (at least, hopefully, they cleanup
and/or simplify).
Uninitialized variable, thanks, Jenkins!
Add a note about WT_ASYNC_OP handles.
Add WT_ASYNC_OP, WT_ASYNC_CALLBACK and WT_TXN_NOTIFY to PREDEFINE.
Merge pull request #1148 from wiredtiger/async-doc

Add a section to docs for async operations, closes #1139.