* fn: common.WaitGroup improvements
*) Split the API into AddSession/DoneSession
*) Only wake up listeners when session count reaches zero.
* fn: WaitGroup go-routine blast test
* fn: test fix and rebase fixup
* fn: sync.WaitGroup replacement common.WaitGroup
agent/lb_agent/pure_runner has been incorrectly using
sync.WaitGroup semantics. Switching these components to
use the new common.WaitGroup() that provides a few handy
functionality for common graceful shutdown cases.
From https://golang.org/pkg/sync/#WaitGroup,
"Note that calls with a positive delta that occur when the counter
is zero must happen before a Wait. Calls with a negative delta,
or calls with a positive delta that start when the counter is
greater than zero, may happen at any time. Typically this means
the calls to Add should execute before the statement creating
the goroutine or other event to be waited for. If a WaitGroup
is reused to wait for several independent sets of events,
new Add calls must happen after all previous Wait calls have
returned."
HandleCallEnd introduces some complexity to the shutdowns, but this
is currently handled by AddSession(2) initially and letting the
HandleCallEnd() when to decrement by -1 in addition to decrement -1 in
Submit().
lb_agent shutdown sequence and particularly timeouts with runner pool
needs another look/revision, but this is outside of the scope of this
commit.
* fn: lb-agent wg share
* fn: no need to +2 in Submit with defer.
Removed defer since handleCallEnd already has
this responsibility.
