Buffer reads from the substituter

This greatly reduces the number of system calls.

src/libstore/local-store.hh

@@ -46,6 +46,7 @@ struct RunningSubstituter
 {
     Pid pid;
     AutoCloseFD to, from, error;
+    FdSource fromBuf;
 };
 
 
@@ -289,6 +290,10 @@ private:
     void startSubstituter(const Path & substituter,
         RunningSubstituter & runningSubstituter);
 
+    string getLineFromSubstituter(RunningSubstituter & run);
+
+    template<class T> T getIntLineFromSubstituter(RunningSubstituter & run);
+
     Path createTempDirInStore();
 
     Path importPath(bool requireSignature, Source & source);