Enhance LLInstanceTracker variants to be more uniform.

For both the (so far unused) generic KEY form and the KEY = T* form, provide
key_iter, beginKeys(), endKeys().
Change instance_iter so that when dereferenced, it gives you a T& rather than
a T*, to be more harmonious with a typical STL container. (You parameterize
LLInstanceTracker with T, not with T*.)
Fix existing usage in llfasttimer.cpp and lltimer.cpp to agree.
For the KEY = T* specialization, add T* getInstance(T*) so client isn't forced
to know which variant was used.
Add unit tests for uniformity of public operations on both variants.