Provide a way to composite two headers

[ikhoon]

When writing headers, merging headers occupies a large part of CPU cycles.

armeria/core/src/main/java/com/linecorp/armeria/internal/common/HttpHeadersUtil.java

Line 46 in 3595dac

public static ResponseHeaders mergeResponseHeaders(ResponseHeaders headers,

Armeria headers are immutable so it is possible to composite the existing headers to avoid the expensive copy operations.

RequestHeaders compositeHeaders = 
  RequestHeaders.wrap(requestheaders, additionalHeaders, defaultHeaders);

[injae-kim]

oh what an interesting issue! I'll handle this. please assign to me~! 🙇

[injae-kim]

FYI) I'm almost done first PR~! (only a few unit-test left). I'll create PR within tomorrow!