Current UserRepository implementation should be improved:

• drop tig_nodes and add 'node' column in tig_pairs
  • we could also add parent_node column in tig_pairs to maintain tree-like structure
  • I'm not sure that we want to normalise those columns (in theory yes - but our past experience shows that this doesn't usually lead to improved performance; on the other hand - having it stored directly would simplify things)
• add lastModified column to tig_pairs to help track changes made to entries (and maybe "value hash"? in a vain similar to current UserRepoRepository detection if value was modified)
• possibly add soft delete column - so we would know that the value was removed (when querying for items "older than")
• deprecate "subnode" methods in 8.x releases

Subsequently, UserRepoRepository should be adjusted and store items individually.

Motivation:

• we already have other tasks that suffers from current implementation (#issue #1119 and #issue #729)
• in the future we are pondering moving configuration to the repository, and having this change would greatly help.