A store that routes each key to the store that owns it
Remarks
This is the one place that turns declared placement into working drivers.
With no manifest on disk it produces a store indistinguishable from the
single local driver the app used before routing existed, so an unconfigured
deployment keeps working unchanged.
Throws
if the manifest exists but is invalid, or declares a driver that is
not supported
Builds the application's
BlobStorefrom the location manifest.