Release v3.18.0

This release includes:
 - Multiple fixes to memory leaks and invalid memory access
 - Improvements in clean command, removing more unnecessary Manifests
 - Adds new flags, keepcache, that keeps all staged cache after an update and
   max-parallel-downloads (renamed from max-pack-parallel-downloads) that lets
   user set the number of parallel downloads for packs and fullfiles.
 - Major improvements in swupd test library. Porting to new testlib was
   concluded and all tests from old library was ported.