Avoid having multiple per-user arrays for RRs, displayname disambiguation etc.

[ara4n]

Instead we should just track one datastructure per room per-user and fill it full of the necessary info, otherwise we are just duplicating all those keys for no reason. Split out from #3307 (comment)

[lampholder]

@ara4n Is this fundamentally a memory-use optimisation?

[turt2live]

This is still relevant, and I don't believe needs more information. Better data structure handling is something we should do.