faster SRCINFO parsing + memory-based building

Switched to parsing srcinfo with channels on all available cpus, speeding
up srcinfo-parsing and queue generation by a lot.

New memory-limit based building will max out the available memory while
not building the same packages at the same time for different marchs,
fixing some long-standing bugs like firefox not building at the same time
because the same ports are used for profile-based optimization. This
also drops the artificial delay on build-start, speeding up things even
more. This also means there is no hard-coded limit on how many packages
can be build at once anymore. As long as there is RAM available, builds will be
started.

flags.yaml

@@ -26,9 +26,9 @@ common:
 
 lto:
   rustflags:
-    - "-Clto=fat"
     - "-Ccodegen-units=1"
-    - "-Clinker-plugin-lto"
 
   options:
-    - "!lto": "lto"
+    - "!lto": "lto"
+
+  cargo_profile_release_lto: "fat"