Minor spike optimisations

Not a comprehensive solution but should hopefully mitigate #691 a bit.

Main changes overview:
-Removed some redundancy from spike collision checks
-Moved spikes into the scenery thinker
-Only retracting spikes run their thinker
-Made spikes solid by default (this doesn't change anything on the user's end, it's just cleaner)

This decreases the number of P_CheckPosition calls at the beginning of CEZ2 by about 300 or so.

pre-compiled exe: srb2stabs2.exe