Src-specific metadata

[rcarver]

This changes how source-specific metadata is stored, ensuring isolation between overlapping data from multiple sources.

From:

type Meta struct {
    Sidecar Content
    Srcs SrcSpecific
}

To:

type Meta struct {
    Src map[index.SrcID] SrcSpecific
}
type SrcSpecific {
    Sidecar Content
   // other fields
}

Previously, Sidecar was pretty ambiguous and could be set from multiple places.

• support old fields as v0
• add migration test