manager: make addTask method thread safe

The code of Manager::addTask() has a comment
about the fact that this function is not thread-safe
and must be called by the same thread calling Manager::pollEvent().
But many part of code seem not taking in account this notice
and call it outside of the correct thread.

This ticket fixes this by using a mutex.

Change-Id: I82d4cc2d6f26c1157caae0e4826dbf96a73561c6
Tuleap: #837