Improve performance

[bfops]

Playform has gotten noticeably laggy. Improve the terrain gen speed, framerate, and load speed.

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5af5c26 - lagginess while moving around has decreased; view updates have two different priority tiers, and the amount of time spent processing each of them is capped.

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72c429e - cap the number of outstanding terrain requests from the client, which reduces a lot of the network congestion

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7a98118 - more granular locking in the server's terrain data structures, so everything doesn't entirely lag out on big computations like generating trees.

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Lagspikes still happen when multiple trees are in the works?

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Duplicate of #151.

Lagspikes still happen when something (e.g. tree generation) is holding onto the SVO lock, but other things try to touch it (e.g. for raycasting). #143 will make this a lot easier to fix.