This changes the way that replica failures are handled such that not all
failures will cause the replica shard to be failed or marked as stale.
In some cases such as refresh operations, or global checkpoint syncs, it is
"okay" for the operation to fail without the shard being failed (because no data
is out of sync). In these cases, instead of failing the shard we should simply
fail the operation, and, in the event it is a user-facing operation, return a
5xx response code including the shard-specific failures.
This was accomplished by having two forms of the `Replicas` proxy, one that is
for non-write operations that does not fail the shard, and one that is for write
operations that will fail the shard when an operation fails.
Relates to #10708
39e7c30912