XAPI UTF-8 string handling

There are multiple issues.

1. Updates to the XAPI database sanitise input strings, but try
generating the notification using the unsanitised input. This
causes the database's event thread to terminate and cease further
processing.

2. XAPI's UTF-8 encoder implements v3.0 of the Unicode spec, but XAPI
uses libraries which conform to the stricter v3.1 of the Unicode
spec. This causes some strings to be accepted as valid UTF-8 by
XAPI, but rejected by other libraries in use. Notably, such strings
can be entered into the database, after which the database can no
longer be loaded.

3. There is no input sanitisation for Map/Set updates on objects in the
XAPI database.