fn: I/O related improvements (#809)

*) I/O protocol parse issues should shutdown the container as the container
goes to inconsistent state between calls. (eg. next call may receive previous
calls left overs.)
*) Move ghost read/write code into io_utils in common.
*) Clean unused error from docker Wait()
*) We can catch one case in JSON, if there's remaining unparsed data in
decoder buffer, we can shut the container
*) stdout/stderr when container is not handling a request are now blocked if freezer is also enabled.
*) if a fatal err is set for slot, we do not requeue it and proceed to shutdown
*) added a test function for a few cases with freezer strict behavior

api/agent/drivers/driver.go

@@ -47,7 +47,7 @@ type WaitResult interface {
 	// provided context is canceled and the container does not return first, the
 	// resulting status will be 'canceled'. If the provided context times out
 	// then the resulting status will be 'timeout'.
-	Wait(context.Context) (RunResult, error)
+	Wait(context.Context) RunResult
 }
 
 type Driver interface {