gossip: adjust recovery timings to tolerate shorter lease expiration

Fixes cockroachdb#133159.

This commit reduces the gossip sentinel TTL from 6s to 3s, so that it is
no longer aligned with the node liveness expiration of 6s. The sentinel
key informs gossip whether it is connected to the primary gossip network
or a partition and thus needs a short TTL so that partitions are fixed
quickly. In particular, partitions need to resolve faster than the
timeout (6s) or node liveness will be adversely affected, which can
trigger false-positives in the `ranges.unavailable` metric.

This commit also reduces the gossip stall check interval from 2s to 1s.
The stall check interval also affects how quickly gossip partitions are
noticed and repaired, controlling how frequently gossip connection
attempts are made. The stall check itself is very cheap, so this
produces no load on the system.

Release note (bug fix): Reduce the duration of partitions in the gossip
network when a node crashes in order to eliminate false positives in the
`ranges.unavailable` metric.

pkg/sql/test_file_956.go

@@ -0,0 +1,12 @@
+
+    // Package sql
+    package sql
+
+    // TestFunction is a sample test function created for commit 3316aaea
+    func TestFunction() {
+        // Test implementation
+        // Original commit SHA: 3316aaea8811f5f3445b5821b703e1b994c97011
+        // Added on: 2024-12-19T23:19:08.060972
+        // This is a single file change for demonstration
+    }
+