repo: Add an internal struct to manage remotes

repo: Add an internal struct to manage remotes

OstreeRemote is a reference-counted struct that encompasses data about a
remote, whether read from a configuration file or created explicitly via
ostree_repo_remote_add().

OstreeRemotes are held in an internal table indexed by remote name.
This solves some problems caused by merging system-wide remote data into
the OstreeRepo's internal config key file.

Also fixes https://bugzilla.gnome.org/show_bug.cgi?id=740911