airframe-http: HttpMultiMap overhead

[xerial]

HttpMutiMap uses a linear search for adding a new entry. It deteriorates the performance with O(H x N) where H is the header size and N is the number of RPC requests.

When there is no header addition, 5M http request objects can be created in a second, but just adding 5 heders makes the performance around 2M request objects/sec.