This commit squashes together some fixes for riak_core claim. There is
a full write up in docs/claim-fixes.md.

In brief: In some situations riak_core_claim can produce rings that
have tail violations. A tail violation is where some preflists (drawn
from the tail of the ring) do not enforce the target-n-val property.
One example is ring using the old claim diag for 16 vnodes and 5
nodes. Such a ring would have a tail of 1. The last two preflists from
that ring would be [5, 1, 1] and [1, 1,2] both of which would mean >1
replica on a single node. These "tail-violations" are addressed.

A further issue with claim was that distributing existing claim to a
new node could result in an unbalanced ring. An example would be
adding a 5th node to a 32 vnode ring of 4 nodes. The distribution went
from [8, 8, 8, 8] to [6, 6, 6, 8, 6] . Before assigning vnodes claim
calculates "deltas" as a measure of how many vnodes each node wants or
should give up. Prior to the change, for this example the deltas were
[2, 2, 2, 2, -6] which means "you can 2 partitions from each of 4
nodes in order to provide 6 on the 5th". Claim would be greedy and
take all 2 from each of the first 3 nodes and stop. This fix balances
the deltas before claim so that they always equal the wants of the
claiming node(s). For the example the deltas would be [2, 2, 1, 1, -6]
ensuring that the final balance of vnodes will be [6, 6, 7, 7, 6].
These "unbalanced rings" are fixed.

NOTE: there may still be ways to get bad rings with "leave" commands,
as this is still not tested

Some mild renaming/refactoring for better reading (I got confused while
figuring out dialyzer errors and these names made more sense when I
re-read it.)