Apply HTTP/2 local settings after ACK is received

Motivation:

Previously local settings is applied on its transmission.  But this
makes settings value out-of-sync with remote endpoint.  The
application of local settings must be done after the remote endpoint
sends back its SETTINGS ACK.  This synchronization is vital to the
protocol.  Suppose that server sends SETTINGS_HEADER_TABLE_SIZE 0 on
connection establishment and resizes its header table size down to 0.
Meanwhile, client sends HEADERS which has header block encoded in
default 4096 header table size, because client has not seen the
SETTINGS from server.  Server tries to decode received HEADERS but due
to the difference of header table size, decoding process may fail.

Modifications:

We don't apply settings on transmission.  Instead, we keep track of
outstanding settings in FIFO queue.  When we get ACK, we pop oldest
outstanding settings and apply its values.

Result:

The outstanding settings values are applied when its ACK is received,
which is compliant behavior to the HTTP/2 specification.