Add an 'init everything' operation to the first backup pass

IBackupTransport.performBackup() now takes a flag "wipeAllFirst", which if set
will result in the entire restore set for the current device/account being wiped
clean prior to the storage of the provided package.  This ensures that a device
on which backup has just been enabled will not confront potentially-stale
information, nor will the restore set potentially contain mismatched data from
orphaned packages.

The Backup Manager has also been revised to pass this flag when first backing up
its master metadata block (and never pass it thereafter unless something has
caused the backup state tracking to be erased, e.g. the user has opted out of
backup and then later re-enabled it).