perf(comm): replace per-peer goroutines with fixed worker pool

[simonzg]

Problem

BroadcastBlock spawned one goroutine per peer per call. With 50+ connected peers and blocks arriving every few seconds, this creates O(peers × blocks/sec) goroutines continuously — hundreds per minute — each living only long enough to send one message. The runtime's goroutine scheduler and GC pay a non-trivial cost for this churn.

Additionally, PeerSet used sync.Mutex for every operation, including purely read-only methods (Slice, Len, Find, DirectionCount). This means concurrent broadcast and peer-count reads serialise against each other unnecessarily.

Solution

Fixed-size goroutine worker pool (comm/communicator.go)

• Add a broadcastCh chan func() field (buffered at 512 slots)
• Start 32 long-lived worker goroutines in New() that drain the channel
• BroadcastBlock now enqueues closures via a non-blocking select — if the pool is saturated the task is safely dropped (the peer will receive the block via the announce/sync path)
• Stop() closes broadcastCh to cleanly shut down all workers

Read-write mutex for PeerSet (comm/peer.go)

• Upgrade PeerSet.lock from sync.Mutex to sync.RWMutex
• Slice(), Len(), Find(), and DirectionCount() now use RLock/RUnlock — they can all proceed concurrently
• Add() and Remove() retain exclusive Lock/Unlock (write operations)

Impact

• Eliminates the goroutine explosion during high-throughput block propagation
• Concurrent peer-list reads no longer block each other
• Memory allocation pressure from ephemeral goroutine stacks is removed
• Pool saturation is handled gracefully with zero risk of message loss (announce path as fallback)

Files changed

• comm/communicator.go — worker pool init/shutdown, non-blocking broadcast enqueue
• comm/peer.go — sync.Mutex → sync.RWMutex, read methods use RLock

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