v0.28.0 * Normalize all Storage.xxx_add() methods to return a summary * cassandra: Add 'check_missing' option, to allow updating objects * cassandra: Add a test of a 'complex' migration, with a PK update * Add a new TenaciousProxyStorage * Make postgresql's origin_add not raise an error in case of conflict * Stop storing authority/fetcher metadata. * tenacious: Document potential issues about objects being dropped * Use swh.core 0.14