SERVER-13951 Split compact up into WriteUnitOfWorks

Each document is its own WUOW and we maintain consistency with every document.
This was the last caller of isCommitNeeded() so that method has been removed.
SERVER-13951 Clean up OperationContext usage in dbtests
Fix repair5.js failures when killOp comes after first file is allocated
Merge remote-tracking branch 'upstream/master'