Investigate setting default HTTP/2 options for upstream clusters

[youngnick]

In #1375, a PR to Envoy suggested setting HTTP/2 options as follows:

    http2_protocol_options:
      max_concurrent_streams: 100
      initial_stream_window_size: 65536 # 64 KiB
      initial_connection_window_size: 1048576 # 1 MiB

This ticket is to investigate setting these.

[youngnick]

xref #1375

[rajatvig]

Would it be ok for me to work on making these as the default? Or is it desired to keep the values configurable?

[sunjayBhatia]

I think the first action item here is to just document for posterity and so we have a sort of decision record what the impact of changing these settings is (what the difference from not setting them is, any memory etc. impacts), and then we can proceed with a change (to change the defaults or make configurable etc.)