tasks #7377: make all tracker migration system events be run in their own process

-also fixing fatal bug infor those who haven't activated fts plugin

Change-Id: I1a12eb7c5bf52c20cf6c2f9cca56d308c9504b52
Merge commit 'refs/changes/37/2937/6' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #7377: make all tracker migration system events be run in their own process
Tuleap 7.4.99.10

Change-Id: Ib14d46a28468e82c42f933457f74ac2abc87c601