SPDY: fix SpdySessionHandler::updateSendWindowSize

This fixes the router so that it no longer violates flow control when
unblocking pending writes.

In Netty 3, downstream writes of SPDY data frames and upstream reads of
SPDY window udpate frames occur on different threads.

When receiving a window update frame, we synchronize on a java object
(SpdySessionHandler::flowControlLock) while sending any pending writes
that are now able to complete.

When writing a data frame, we check the send window size to see if we
are allowed to write it to the socket, or if we have to enqueue it as a
pending write. To prevent races with the window update frame, this is
also synchronized on the same SpdySessionHandler::flowControlLock.

In Netty 4, upstream and downstream operations on any given channel now
occur on the same thread. Since java locks are re-entrant, this now
allows downstream writes to occur while processing window update frames.

In particular, when we receive a window update frame that unblocks a
pending write, this write completes which triggers an event notification
on the response, which in turn triggers a write of a data frame. Since
this is on the same thread it re-enters the lock and modifies the send
window. When the write completes, we continue processing pending writes
without knowledge that the window size has been decremented.