SL-92 WIP - More consistent enforcement of min/max hover values. Setting is stored as a per-account F32 value.

MAINT-4448 FIXED Avatar often stops animating when being animated by an intan solo dance ball on Interesting based viewers only.

MAINT-4106: Correcting crash on exit behavior during normal shutdown.  With the more comprehensive texture cleanup going on, this code was causing an crash on exit when gAgentAvatarp was null.  Adding a check for null before proceeding.

Reviewed with Simon

SH-3455 WIP - moved setAvatarObject() to run after the self av constructor. Disabled sendAgentWearablesUpdate.

Backed out revision 9038e63bc38d, which added viewer version information to what is sent in an appearance metric message. We are taking another approach to get the same information in a more consistent, reliable way.

continued conversion to units system
made units perform type promotion correctly and preserve type in arithmetic
e.g. can now do LLVector3 in units
added typedefs for remaining common unit types, including implicits

added convenience types for units F32Seconds, etc.

Reviewed by Kelly