RedisConnection#Connect(): get rid of spin lock

[Al2Klimov]

Instead of IoEngine::YieldCurrentCoroutine(yc) until m_Queues.FutureResponseActions.empty(), async-wait a CV which is updated along with m_Queues.FutureResponseActions.

👍 Re-connect still works on Redis restart, even if latter interrupted an outgoing message!

[2024-12-06 10:24:02 +0100] critical/IcingaDB: Error during sending query 'XADD' 'icinga:stats' 'MAXLEN' '1' '*' 'ApiListener' '{"perfdata":[{"counter":false,"crit":null,"label":"api_num_co...' ... which has been fired and forgotten: Broken pipe
[2024-12-06 10:24:02 +0100] information/IcingaDB: Connected to Redis server

ref/NC/820479
claimed they start Icinga 2 with Icinga DB enabled – and it OOMs. I hope this spin lock is not responsible, but better safe than sorry.

Also, I'm sure we all agree that spin locks should be avoided, at least where easily possible.

• Don't generate a mutex for each Locked<T>, share one per object #10117 (comment)
• CpuBoundWork#CpuBoundWork(): don't spin on atomic int to acquire slot #9990