fn: sync.WaitGroup replacement common.WaitGroup (#937)

* 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.

api/agent/pure_runner.go

@@ -671,7 +671,7 @@ func DefaultPureRunner(cancel context.CancelFunc, addr string, da DataAccess, ce
 }
 
 func NewPureRunner(cancel context.CancelFunc, addr string, da DataAccess, cert string, key string, ca string, gate CapacityGate) (Agent, error) {
-	a := createAgent(da, true)
+	a := createAgent(da, true, nil)
 	var pr *pureRunner
 	var err error
 	if cert != "" && key != "" && ca != "" {