WIP Threadpool merge scheduler

[albertzaharovits]

Highlights:

• there is one merge scheduler per shard
• for IO throttling, across shards, this uses static variables (the set of "active" merges)
• there is no merge size cutoff for IO throttling, all merges are IO throttled (except for forced merges, and merges when the shard is closing)
• the max merge threads setting is respected per scheduler (per shard)
• merges are ordered by size per scheduler (per shard), smaller ones are handled first
• new merges throttling is not implemented (i.e. to prevent new merges comming in), just some hooks are exposed (that the Internal Engine implements)

[albertzaharovits]

We've ultimately merged a different alternative: #120869