Multi-Threading Consideration for HTTP Library

By default this library is not thread safe, for instance HTTP listeners and HTTP messages are not thread safe. Exceptions are:

HTTP client is Kea thread safe (i.e. is thread safe when the multi-threading mode is true).
date time is thread safe (mainly because its encapsulated POSIX time is private and read-only, or because all methods are instance const methods or class methods).
URL is thread safe (all public methods are const methods).