Add automatic garbage collection

Nix can now automatically run the garbage collector during builds or
while adding paths to the store. The option "min-free = <bytes>"
specifies that Nix should run the garbage collector whenever free
space in the Nix store drops below <bytes>. It will then delete
garbage until "max-free" bytes are available.

Garbage collection during builds is asynchronous; running builds are
not paused and new builds are not blocked. However, there also is a
synchronous GC run prior to the first build/substitution.

Currently, no old GC roots are deleted (as in "nix-collect-garbage
-d").

doc/manual/release-notes/rl-1.12.xml

@@ -393,6 +393,10 @@ configureFlags = "--prefix=${placeholder "out"} --includedir=${placeholder "dev"
     package repository.</para>
   </listitem>
 
+  <listitem>
+    <para>Automatic garbage collection.</para>
+  </listitem>
+
 </itemizedlist>
 
 <para>This release has contributions from TBD.</para>