Prevent unnecessary locking when adding group lock handler

[ming]

pj_grp_lock_add_handler() will lock the group lock first, which has a side effect of potentially establishing an incorrect lock ordering when using thread detector tool such as Helgrind.

We should use pj_grp_lock_create_w_handler() instead that adds the handler during creation and doesn't require acquiring the lock.

Also see ticket #1821

[ming]

In 6136:

Closed #2261: Prevent unnecessary locking when adding group lock handler