Barrage: apply redirection mapping is broken into disjoint groups

[nbauernfeind]

Fixes #2938

Redirection mapping is performed in four distinct groups:

1. adds that are new adds
2. mods that replace rows scope into a subscribed viewport
3. mods that propagate as adds (the adds have not yet been propagated, but this modifies an add)
4. mods that are just mods

Each one share the ArrayList of redirection mappings. Existing logic only would advance to the next mapping if the key matches the last row of that chunk -- however that row is only in one of the four groups. I now skip until I find the appropriate chunk that should be modified.

[rcaudy]

Review pass for row offset changes. We're going to need to go over BarrageStreamGenerator more carefully.