release-19.2: allocator: fix incorrect rebalance signal and target

[andy-kimball]

Backport 1/1 commits from #43041.

/cc @cockroachdb/release

---

This change fixes two bugs:

• The first bug is related to the comparison of floating point
  numbers when considering to rebalance a range. The diversity score
  of two identical ranges (even the same range) may differ by a small
  amount. The fix is to consider candidates equal if their diversity score
  have very close values (within 1e-10).
• The second bug has been introduced with
  *: remove the remaining uses of ReplicaDescriptors.Unwrap #39157 and resulted in
  ignoring the newly added replica when determining what existing replica
  it should replace in Allocator.simulateRemoveTarget.
  As a result - when the first bug was triggering and unnecessary
  rebalancing was happening - the replica to be dropped was chosen
  at random (in the case where all existing replica are necessary).
  For example - let's say a range is constrained to have a replica in each
  of the regions A,B,C. We are trying to add a new replica in C.
  Evaluating what should be removed from the A,B,C may choose any one of them.
  If the choice is A or B, the constraint will be violated.
  If we however ask the same question for A,B,C(old),C(new) - the result
  will be C(old) as this will preserve the constraint (to have one
  replica in each region).

Fixes #42715
Fixes https://github.com/cockroachlabs/support/issues/348

Release note (bug fix): the allocator now considers stores with very close
diversity scores equal (all other things being the same) and doesn't attempt
to rebalance.
Release note (bug fix): the allocator considers the new store being added
when looking for target in case of rebalance.

[cockroach-teamcity]

This change is 

[dt]

I'm going to wait on restarting 19.2.2 qualification to pick this one up too.

[ajwerner]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained