Maybe make a custom Http2HeaderEncoder

Currently the default Netty Http2HeaderEncoder grabs an iterator from an Http2Headers objects and iterates over the values, adding them to the table and serializing them to a ByteBuf. We could save on a lot of allocations and cputime if we could pass an array directly in and skip Iterator. We don't even have to deal with the hpack portions of this since we can reuse that API independent of the Http2HeaderEncoder interface.