Re-balance RCS thrusters if CoM moves too much.
RCS balancing CoM hacks.
Fix spurious CoM shifts.

The coordinate system is reset periodically, and once in a while that
meant using an old value for VesselState.CoM and a new value for the
root part's position.
Fix the RCS "task count" debug infoitem.
Reduce RCS solver precision for a big speed boost.

This reduces execution time by 90% on a ship with 56 4-way RCS blocks,
with no noticeable loss in accuracy.
Make RCS exhaust effects match balanced throttles.