Fix updateValidatorDistInfoFromPool (#3046)

Fixes regression introduced by #2984.
Continuiation of #3033 , which didn't fix the simulation issues.
(candidate) Complete solution for #3019, 9002 halt bug.

From #2984, it isn't sufficient to take the fee pool rewards of a validator. Since we don't track delegator accums (as we do with validator accums), and because onValidatorModified >updateValidatorDistInfoFromPool is also being called upon delegation updates (or at least I believe this is the reason), it is necessary to also withdraw self delegation.

TODO: I don't think self-delegation should be required to be modified here... consider using a delegation hook to do the self-delegation withdraw part instead, e.g. splitting the updateValidatorDistInfoFromPool function into two. It might not result in cleaner code, however. Think hard.

types/decimal.go

@@ -400,6 +400,11 @@ func (d Dec) TruncateInt() Int {
 	return NewIntFromBigInt(chopPrecisionAndTruncateNonMutative(d.Int))
 }
 
+// TruncateDec truncates the decimals from the number and returns a Dec
+func (d Dec) TruncateDec() Dec {
+	return NewDecFromBigInt(chopPrecisionAndTruncateNonMutative(d.Int))
+}
+
 //___________________________________________________________________________________
 
 // reuse nil values