This mechanism existed but was deleted. We bring it back in order to
prevent lower priority streams from starving.

Previously we use largest stream ID received so far as last-stream-ID,
and it is irrevant that it is passed to the callback (thus upper layer).
Now the stream ID which is passed to callback is eligible to
last-stream-ID.

This commit also changes SPDY's flow control size. Previously,
the size for SPDY is the same amount of bytes with HTTP/2.
For example, --frontend-http2-upstream-window-bits=N,
the window size is 2**N - 1. Now SPDY code uses 2**N.

The specification now says that index to the header table entry is
1-based. Since 0-based index is very handy to access arrays, we
internally uses 0-based index. We just convert it to 1-based when
we emit to the block and convert 1-based to 0-based on decoding.

Remove sorting headers from library code. The application must sort
them if necessary. nghttpx and nghttpd do the sorting of the headers
in stable way if names are equal.