WIP to preserve GUIDs in custom MultiPara fields
WIP to preserve GUIDs in custom MultiPara fields

First attempt at preserving Guids in LfStringField. Appears to work, but
unit tests fail if line 178 of ConvertMongoToFdoCustomField.cs is
commented out (line 178 is the "no change in contents? skip" shortcut).
So round-tripping really isn't working yet.

sillsdev/LfMerge,ermshiperete/LfMerge,ermshiperete/LfMerge,ermshiperete/LfMerge,sillsdev/LfMerge,sillsdev/LfMerge