It is very hard to support multiple protocols in backend while
retaining multiple mode settings.  Therefore, we dropped modes except
for default and HTTP/2 proxy mode.  The other removed modes can be
emulated using combinations of options.  Now the backend connection is
not encrypted by default.  To enable encryption on backend connection,
use --backend-tls option.

nghttpx: Deprecate backend-http1-connections-per-frontend in favor of backend-connections-per-frontend

This is required to avoid session stall because of too slow frontend
connection.

We added 2 new option instead: --frontend-http2-max-concurrent-streams
and --backend-http2-max-concurrent-streams.

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.

This reverts commit 8aac5d6a.

This reverts commit dbffb899.