[#2656] Minimize ByteBuf.writeBytes(...) calls by pack the separator into the HttpHeaderEntity.

Motivation:
Currently we do 4 ByteBuf.writeBytes(...) calls per header line. This is can be improved.

Modification:
Introduce two new HttpHeaders methods to allow create HttpHeaderEntity which contains the separator. With this we can minimize it to 2 ByteBuf.writeBytes(...) calls per header line

Result:
Performance improvement.