By definition this is not 'Mutable state of a cell' and does not belong in CellStore.
This change should improve startup times (graph is now loaded on demand) and edits to 'pathgrid.hpp' no longer cause the entirety of OpenMW to be rebuilt.
Previously, whenever a single attribute value changed for a player, that player then sent a PlayerAttribute packet with all values for all 8 attributes.
This did not cause anywhere as much packet spam as PlayerSkill used to, but there was no good reason not to fix it as well.
Previously, whenever a single skill value changed for a player, that player then sent a PlayerSkill packet with all values for all 27 skills, plus the player's progress towards the next level and the bonuses to each attribute on the next level up as the result of sklll increases thus far.
This commit makes PlayerSkill contain only the values of specific skills, moves the player's progress towards the next level to PlayerLevel packets, and moves the bonuses to each attribute on the next level up to PlayerAttribute packets.
Players now also send a PlayerSkill packet whenever their progress towards a new point in a skill changes. This was previously avoided so as to not have massive packet spam.
Previously, an attempt by the server to simultaneously change a player's cell and skills (as you'd expect when a player file is loaded) led to:
1) The server sending the cell packet first and the skill packet afterwards
2) The player receiving the cell packet and sending their own skill packet as part of the client's forced skill update
3) The player receiving the skill packet from the server
4) The server receiving the skill packet from the player
The result was that, if the player then left the server without sending another skill packet, the server's memory retained the skills the player had sent instead of the skills it had sent to the player.
This is the first step in a solution to that situation and similar ones.
