[SYCL][PI][L0] Retain events in event wait list until command has completed. (#3008)

This change implements code to do a retain on any event put into
a level zero wait list.  The events in the wait list will be
released after the command that was waiting on them has finished
execution as indicated by the command's completion event having
been signalled.  This code also adds an internal environment
variable that allows filtering of already signalled events out of
the wait list, which means they do not have to be retained, and
also may have some positive performance impacts.  This change
set does not filter the event wait list by default.

Author: kbsmith-intel