Replace hard-coded minimal bound on average load in tempered LB

[ppebay]

What Needs to be Done?
The decision to do load-balancing in the tempered algorithm is currently constrained by hard-code minimum bound on average load (set at 1e-10).

Is your feature request related to a problem? Please describe.
There are no first principles justifying this and this could also result in incorrect decisions.
For example, object loads could be very small and result in a balance smaller (on average) than this hard-coded value -- while imbalance would nevertheless be high and warrant LB triggering.

Describe potential solution outcome
A replacement of this hard-coded value using first principles should be thought of, and implemented.

Describe alternatives you've considered

Additional context

[nlslatt]

Given the extremely small magnitude, I suspect this was originally intended to avoid load balancing when there effectively was zero load.