Before this fix, when a
- wait
- stage
- statement
- transaction (in 5.7)
was still executing, respective tables
- performance_schema.events_waits_current
- performance_schema.events_stages_current
- performance_schema.events_statements_current
- performance_schema.events_transactions_current (in 5.7)
would display the event with
- TIMER_START populated
- TIMER_END as NULL
- TIMER_WAIT as NULL

This makes it difficult to find for how long an event not yet completed has
been running.

With this fix,
- TIMER_START is populated (unchanged)
- TIMER_END is populated with the current timer value (NOW)
- TIMER_WAIT is populated with the time elapsed so far (TIMER_END -
  TIMER_START)

Monitoring applications can simply query
  WHERE (END_EVENT_ID IS NULL) and (TIMER_WAIT > X)
to find out events that are:
- not yet completed (no END_EVENT_ID)
- taking longer than X picoseconds already.