adjust hysteresis to avoid initial over-deposit incentive

[djrtwo]

Address #1609

Make hysteresis calculations configurable.
Change default increment/decrement thresholds to x-0.25 and x+1.25

[protolambda]

From chat earlier, we need to consider that if we do not offset the actual effective balance, we:

• increase allowance for mistakes by 0.25
• increase punishment by requiring another 0.25 extra

Previously:
Downwards hysteresis width: 32-32 = 0
Upwards hysteresis width: 32.5-31 = 1.5
Now:
Downwards hysteresis width: 32-31.75 = 0.25
Upwards hysteresis width: 32.75 - 31 = 1.75

I think the system is closer to original intention if we change it to:

• A small margin of e.g. 0.05 for initially offline validators to not immediately lose 1 effective balance increment because of some small inactivity.
• The old 1.5 upwards width.

[mcdee]

What is the rationale behind x+0.75 rather than x+0.25? A +/-0.25 would retain the current gap of requiring 0.5 Ether in rewards (or additional deposit) prior to an uptick in effective balance, and solves the issue of newly on-boarded deposits not suffering an immediate decrease in effective balance if they miss their first attestation.

[vbuterin]

Yeah, it should be -0.25 and +1.25, not +1.75.

Downwards hysteresis width: 32-31.75 = 0.25
Upwards hysteresis width: 32.75 - 31 = 1.75

The error here is that as implemeneted the "downwards hysteresis width" (wrong term arguably) is actually negative 0.25, and not 0.25. So preserving the 1.5 width would require an upwards threshold of 1.25.

[protolambda]

Yeah the term could have been better. I care more about the hysteresis numbers than the names though, the leap needed to get back to 32 is affected significantly.

[djrtwo]

modded to be 0.25/1.25

Ready for review and merge @protolambda

[protolambda]

With experimentation and review these numbers may change again, I am thinking we could just make it configurable, and avoid further spec changes.

[djrtwo]

okay, now configurable @protolambda

[protolambda]

LGTM

[protolambda]

So previous behavior is, for context:

HYSTERESIS_QUOTIENT: 2
HYSTERESIS_DOWNWARD_MULTIPLIER: 0
HYSTERESIS_UPWARD_MULTIPLIER: 3

[Sintayew4]

#1627