carddav: allow backend to handle addressbook-multiget

[emersion]

Right now addressbook-multiget request will result in many GetAddressObject calls. This is not great for backends which need to fetch resources from a remote server.

[prasad83]

Let CardDAV Backend interface expect GetAddressObjects(context.Context, paths []string, *AddressDataRequest) (map[string]*AddressObject, error)

Now carddav server can GetAddressObjects and iterate over it to process further.
Backend will now get more control on single fetch vs multiple fetch based on its logic.

@emersion - Would you like a PR for this? Let me know.

[emersion]

Something like this, yeah. I don't think we need the return value to be a map - a slice would be enough.

[emersion]

Same applies to CalDAV too.

[prasad83]

@emersion - Please review PR #194

Slice based return expectation defined in respective Backend interfaces.

caldav/server.go

GetCalendarObjects(context.Context, paths []string, *CalendarCompRequest) ([]CalendarObject, error)

carddav/server.go

GetAddressObjects(context.Context, paths []string, *AddressDataRequest)([]AddressObject,error)

handleMultiGet delegates call to this method, does lookup for response preperation.