repo: Use per-transaction staging dir

repo: Use per-transaction staging dir

Concurrent pulls break since we're sharing the staging directory for
all transactions in the repo. This makes us use a per-transaction directory.

However, in order for resumes to work we first look for existing
staging directories and try to aquire an exclusive lock for them. If
we can't find any staging directory or they are all already locked,
then we create a new one.

https://bugzilla.gnome.org/show_bug.cgi?id=757611