HTTP/2: workaround for clients which fail on table size updates. There are clients which cannot handle HPACK's dynamic table size updates as added in 12cadc4669a7 (1.13.6). Notably, old versions of OkHttp library are known to fail on it (ticket #1397). This change makes it possible to work with such clients by only sending dynamic table size updates in response to SETTINGS_HEADER_TABLE_SIZE. As a downside, clients which do not use SETTINGS_HEADER_TABLE_SIZE will continue to maintain default 4k table.