Backport object size/sibling limits to 1.4

* Objects larger than warn_object_size will log a warning on read or
  write. The default is 5MiB
* Objects larger than max_object_size will fail to be written, logging
  an error message. By default no limit.
* Objects with a sibling count larger than warn_siblings will log a
  warning on write. By default 25.
* Objects with a sibling count larger than max_siblings will fail to
  write, logging an error message. By default no limit.

Note: in 2.0, there will be default values for max size and siblings, but I didn't want to risk failing operations for users in a point release.