Allow multiple xattrs to be obtained in a lookupin

At present it is only possible to obtain a single xattr per lookupin operation but we would like the ability to fetch multiple.

Use case:
Sync Gateway is adding a feature to allow a user to access a user defined xattr within their Sync Function. In order to get this we would have to add another lookupin operation as we already get the "_sync" xattr in an existing lookup operation. This means we are doubling our lookups which is too costly for us.

Tagging Adam Fraser