SyncRepos: manage global variables

After calling load_emerge_config, use _reset_legacy_globals to cleanly
destroy existing portage.db / settings instances. Then update
portage.db and portage.settings to refer to the new instances.

Also, instantiate a local instance of SyncManager, so that we can
remove the global instance that was previous accessed via get_syncer.
There's no reason to have a global instance, and global instances are
a maintenance nightmare (see portage.db and portage.settings changes
above).