Copy instrument target data in shadow_copy_event()

Add instrument target data to ust_app_event struct.

In delete_ust_app_event(), free the copied instrument target data,
and also remove a redundant NULL check.
Add instrument target data to find_ust_app_event()

Add instrument target data to the prototype of find_ust_app_event()
and wherever it is called.

In ht_match_ust_app_event(), use instrument target as one of the
matching criteria.

Add instrument target as a key in add_unique_ust_app_event().