add support for repairCursor command with test coverage
update comments and docstrings for repairCursor
improvements to docstring on Repair() method
Add session.Repair.

Contributed by Mike O'Brien.
Move Repair method out of Pipe implementation.
Properly return ErrCursor on cursor timeouts.

Fix by Daniel Gottlieb.
Enable replica set name assertion in server discovery
Don't use cluster.err
Check live servers are correct
Remove extraneous server check
Improve ReplicaSetName field doc, drop dup tests