Added aliases for std::chrono typedefs (eg. 'Seconds' for
std::chrono::seconds). These aliases are used together with templated
helper functions to replace time_t with std::chrono::seconds for most
cases, in particular for 'modified' and 'expires' values in Response.
When we get a request with an explicit expiration time, we're now starting a timer, and will trigger a refresh once the data expires. This gives requesters a chance to update their data.
