SERVER-15541 Clean up some usages of WriteUnitOfWork

Removes some WriteUnitOfWork objects where they are unneeded, and
moves the declaration of some existing WriteUnitOfWork objects to
limit their scope to the write being performed (so that
yield-capable reads are not inside them).
SERVER-13627 Test that replSetReconfig prohibits 0-vote arbiter.

Closes #856

Signed-off-by: Benety Goh <benety@mongodb.com>
SERVER-7596 Make authSchemaVersion 28Final if no user docs
Merge remote-tracking branch 'upstream/master'