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.

config_dist.yaml

@@ -62,8 +62,10 @@ build:
   worker: 4
   makej: 8
   checks: true
-  # builds over this threshold are considered slow (in cpu-time-seconds)
-  slow_queue_threshold: 14400.0
+  # how much memory ALHP should use
+  # this will also decide how many builds will run concurrently,
+  # since ALHP will try to optimise the queue for speed while not going over this limit
+  memory_limit: "16gb"
 
 logging:
   level: INFO