Adds a new info record to the summary.
If the force_keep_uid is false, then a new uid is automatically re-assigned by calling
next_uid_string. It's an error to use force_keep_uid
when the info has none set.
The this adds its own reference to info, if needed, and any previously loaded info is
replaced with the new one.
| this |
a FolderSummary object |
| info | |
| force_keep_uid |
whether to keep set UID of the |