Add BattleStats modifier map with source attribution

[mysterymagination]

• allows for intelligent aggregate add/remove of stat mods i.e. with one effect building on the current set of effects' modifiers rather than overwriting their modifiers.
• this would ideally live in BattleStats since it doesn't need to know anything about the IBattleStatsBearer impl

[mysterymagination]

fixed somewhere around 4ad15eb although, the tossaway timerhandle may come back to bite me if TimerManager actually wants to use it for something...