ShardedDaemonProcess: throttle keep-alive messages and limit to subset of nodes

[pjfanning]

Ports akka/akka-core#31837. Keep-alive pings were sent from every node on a fixed timer, causing thundering herd issues and unnecessary load (e.g. waking all projection instances simultaneously).

Part of #2730

Settings (ShardedDaemonProcessSettings)

• Added keepAliveFromNumberOfNodes: Int (default: 3) — only the first N sorted cluster members send keep-alives, making selection deterministic across nodes
• Added keepAliveThrottleInterval: FiniteDuration (default: 100 ms) — inter-message delay when sending keep-alives
• Both settings exposed via Scala/Java with* APIs

KeepAlivePinger behavior

• Identities sorted for deterministic ordering across nodes
• If node is already MemberStatus.Up at actor start, fires Tick immediately instead of waiting for SelfUp
• isActive() check gates whether this node participates in keep-alive at all
• Keep-alive sending is now async via Source.throttle + pipeToSelf, so the next timer fires only after the current batch completes

Config (reference.conf)

pekko.cluster.sharded-daemon-process {
  keep-alive-from-number-of-nodes = 3
  keep-alive-throttle-interval = 100 ms
}