tsm imports rr directly for its non-merge fallback -- investigate

[crandles]

tsm is the only ALB mechanism that imports another mechanism. In pkg/backends/alb/mech/tsm/time_series_merge.go:

nmh, _ := rr.New(nil, nil)
nmpm, _ := nmh.(types.PoolMechanism)

no other mechanism is imported by another mechanism. this isn't inherently wrong, but is somewhat unexpected from the package layout.

Worth understanding before changing:

• is round-robin specifically the right default for the non-merge path, or just the simplest dispatcher on hand?
• are there cases where the non-merge path should match the operator's chosen primary mechanism rather than always being rr?

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To Do:

• research alternatives