Remove max_old_space and use as much as we can

process.memoryUsage() is our friend. also, with --expose-gc we
can use global.gc(). we don't use it yet though

part of #626

src/scripts/backupupload.js

@@ -1,5 +1,5 @@
 #!/bin/bash
-':' //# comment; exec /usr/bin/env node --max_old_space_size=300 "$0" "$@"
+':' //# comment; exec /usr/bin/env node --expose-gc "$0" "$@"
 
 // to understand the above hack read http://sambal.org/2014/02/passing-options-node-shebang-line/