Nerf mammoth missiles

Mammoth missiles are a bit crazy, especially vs. buildings. Nerfing them a bit.
To compensate, increasing their spread a bit.

To increase missile efficiency (avoid extra missiles being wasted on overkill), their damage has been reduced from 75 to 45. They can still kill orcas in 2 shots.
To further avoid wasteful firing of missiles, the burst delay was increased, and the speed of the missiles was increased.