reorder struct items for alignment

On 32-bit platforms, use of atomic.* 64-bit functions needs to be
careful to ensure 64-bit alignment. This was a problem in the
rackAwareRR struct, causing panics on 32-bit platforms.
https://pkg.go.dev/sync/atomic#pkg-note-BUG notes that the first word in
a struct is always 64-bit aligned, to moving the struct member that
causes the issue to the beginning of the struct solves the problem.
See #1666.

Author: satta

policies.go

@@ -904,10 +904,14 @@ func (d *dcAwareRR) Pick(q ExecutableQuery) NextHost {
 // a different rack, before hosts in all other datercentres
 
 type rackAwareRR struct {
+	// lastUsedHostIdx keeps the index of the last used host.
+	// It is accessed atomically and needs to be aligned to 64 bits, so we
+	// keep it first in the struct. Do not move it or add new struct members
+	// before it.
+	lastUsedHostIdx uint64
 	localDC         string
 	localRack       string
 	hosts           []cowHostList
-	lastUsedHostIdx uint64
 }
 
 func RackAwareRoundRobinPolicy(localDC string, localRack string) HostSelectionPolicy {