[core] Replace time_t with std::chrono::seconds

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.

src/mbgl/storage/http_request_base.cpp

@@ -5,18 +5,15 @@
 
 namespace mbgl {
 
-int64_t HTTPRequestBase::parseCacheControl(const char *value) {
+Seconds HTTPRequestBase::parseCacheControl(const char *value) {
     if (value) {
         const auto cacheControl = http::CacheControl::parse(value);
-
         if (cacheControl.maxAge) {
-            return std::chrono::duration_cast<std::chrono::seconds>(
-                       std::chrono::system_clock::now().time_since_epoch()).count() +
-                   *cacheControl.maxAge;
+            return toSeconds(SystemClock::now()) + Seconds(*cacheControl.maxAge);
         }
     }
 
-    return 0;
+    return Seconds::zero();
 }
 
 }