Processing events from upstream connection can result in sending queued frames
from other streams. In this case such streams were not added to handling queue
and properly handled.
A global per connection flag was replaced by a per stream flag that indicates
currently sending stream while all other streams can be added to handling
queue.
With this change all such frames will be added in front of the output queue, and
will be sent first. It prevents HOL blocking when response with higher priority
is blocked by response with lower priority in the middle of the queue because
the order of their SYN_REPLY frames cannot be changed.
Proposed by Yury Kirpichev.
