Package tagUser "Every package should have a package comment, a block comment preceding
the package clause.
For multi-file packages, the package comment only needs to be present in one file, and any
7ne will do. The package comment should introduce the package and provide information
relevant to the package as a whole. It will appear first on the godoc page and should set
up the detailed documentation that follows."
GetKeysByUserOrTagKey returns the user keys as a slice if the tag key is provided
or returns the tag keys as a slice if user key is provided and also an error.
TagUser datastore: ",noindex" causes json naming problems !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Encoded tag key is key's stringID and user key is the parent key.