perf: use last-pair for the single-node commit-index tail access

ober

94288d71e1b263390c74d226bbe731a37118dc47

diff --git a/lib/jerboa-db/cluster.ss b/lib/jerboa-db/cluster.ss
index 78a77a4..0c46131 100644
--- a/lib/jerboa-db/cluster.ss
+++ b/lib/jerboa-db/cluster.ss
@@ -93,7 +93,7 @@
               (let ([log (raft-log leader)])
                 (if (null? log)
                     0
-                    (log-entry-index (car (reverse log)))))
+                    (log-entry-index (car (last-pair log)))))
               (raft-commit-index leader))
           0)))