44661: kv: refresh less and retry more r=andreimatei a=andreimatei

Before this patch, when the DistSender would split a batch into multiple
sub-batches and one of the sub-batches fails, it would collect responses
for the successful ones and return them together with the error. This
used to be pretty important before we had write idempotency, because it
allowed the span refresher to only retry an EndTxn without also retring
other writes in that batch (which would have failed).

Since we've gotten idempotency in the meantime, we can retry those other
writes. In fact, it's arguably better to do it: there's a tradeoff
between refreshing and retrying. Currently the span refresher needs to
refresh the read spans of the successful sub-batches, which refresh is
at risk of failing under contention.

This patch makes the span refresher retry the whole batch without
considering partial successes. With this patch, refreshing the partial
successes is no longer needed because we'll retry those requests. In
other words, we'll refresh less and retry more.

The existing policy of refreshing more and retrying less will start to be
applied inconsistenly with #44654, where we start refreshing when the
client sees a WriteTooOld flag - but we're forced to refresh the whole
batch.

Besides the rationalizations above, this patch allows us to simplify
code by not having to deal with both responses and errors. We can thus
get rid of the enthralling comment on the client.Sender.Send() stating:
"
// The contract about whether both a response and an error can be
// returned varies between layers.
"

Release note: None

Co-authored-by: Andrei Matei <[email protected]>