smp-server: fewer threads and no retained TLS record per client

[shumvgolove]

Each client connection kept memory for its whole lifetime that it did not need:

• Last received TLS record. tls 1.9 updates its receive state lazily, so every connection kept a reference to the last ~16 KB record until the next one arrived. recvTLS now forces the state after each recvData. This applies to all TLS connections, including clients and proxy-to-relay connections.
• Two send threads. Command responses and delivered messages had separate sending threads with a send lock. One send loop now handles both.
• Per-client inactivity thread. Each client had its own thread to disconnect it when inactive. One server-wide thread now sweeps all clients; Client gets a closeTransport action. New test: "Inactive clients".

Threads per connection: 6 → 4.

Server live heap per connection (prodmix: 2000 connections, 22 subscriptions each, traffic):

	Before	After
busy, -ki2k	111.5 KiB	89.8 KiB
busy, default RTS	236.6 KiB	210.5 KiB
idle, -ki2k	90.2 KiB	68.6 KiB

About 0.8 GiB less at 40k connections. Details: docs/smp-server-memory.md on sh/fix-leak, finding 4.