Just like regular indexations, fulltext indexations are subject to last-commit-wins.
Which means that it is possible to reach a state where the fulltext table is
persistently desynchronised from ZODB:
- start fulltext indexation activity on many documents (typically: 100)
- modify one of the documents being indexed
- start fulltext indexation activity caused by this edit, and assume indexation only
happens for this object
- commit the single-object indexation (because it is very fast to retrieve fulltext
data from just one document)
- commit the many-objects indexation later (because it is much slower to
retrieve 100 fulltext representations)

As a consequence, cod emust spawn one fulltext indexation activity per
document, each with the appropriate serialisation tag. Serialisation tag which
must not conflict with regular indexation, so use a fixed prefix.
As a consequence of having to spawn one activity per document, use a
grouping method to still index by batches to amortise transaction overhead.
Keep the same method_id as before for backward-compatibility (maybe
dependencies on this value exist, even though it is bad practice).
Rewrite SQLCatalog_deferFullTextIndexActivity so ot works as a grouping
method, simplifying it in the process:
- build parameter_dict with all entries, as we already know all needed keys
- None is not callable, so test "not None" in just one expression
- remove whitespace at end of line
- use GroupedMessage API