Previously, we use one Http2Session object per DownstreamAddrGroup.
This is not flexible, and we have to provision how many HTTP/2
connection is required in advance.  The new strategy is we add
Http2Session object on demand.  We measure the number of attached
downstream connection object and server advertised concurrency limit.
As long as former is smaller than the latter, we attach new downstream
connection to it.  Once the limit is reached, we create new
Http2Session object.  If the number lowers the limit, we start to
share Http2Session object again.