Eliminate the "store" global variable

Also, move a few free-standing functions into StoreAPI and Derivation.

Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)

src/libutil/util.hh

@@ -413,4 +413,14 @@ string base64Encode(const string & s);
 string base64Decode(const string & s);
 
 
+/* Get a value for the specified key from an associate container, or a
+   default value if the key doesn't exist. */
+template <class T>
+string get(const T & map, const string & key, const string & def = "")
+{
+    auto i = map.find(key);
+    return i == map.end() ? def : i->second;
+}
+
+
 }