Prevent infinite loop if enumerator is applied concurrently to many iteratees
BSONDBPointer.objectId
GridFS: restore enumerate behavior. Fixes #127
[#5] ReadPreferences support (excluding tag support)
Fix GetLastError to match the spec.

http://docs.mongodb.org/manual/reference/command/getLastError/#dbcmd.getLastError
[#5] Tags in ReadPreferences support. Fixes #5