lsa: avoid unnecessary segment migrations during reclaim
lsa: avoid unnecessary segment migrations during reclaim

segment_zone::migrate_all_segments() was trying to migrate all segments
inside a zone to the other one hoping that the original one could be
completely freed. This was an attempt to optimise for throughput.

However, this may unnecesairly hurt latency if the zone is large, but
only few segments are required to satisfy reclaimer's demands.
Message-Id: <5504f579e042af28dad6a9e1e38555deb9881310@scylladb.com>

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