lsa: Fix performance regression in eviction and compact_on_idle
lsa: Fix performance regression in eviction and compact_on_idle

Region comparator, used by the two, calls region_impl::min_occupancy(),
which calls log_histogram::largest(). The latter is O(N) in terms of
the number of segments, and is supposed to be used only in tests.
We should call one_of_largest() instead, which is O(1).

This caused compact_on_idle() to take more CPU as the number of
segments grew (even when there was nothing to compact). Eviction
would see the same kind of slow down as well.

Introduced in 11b5076b3cfa2569077fbbc0e16c2e1a7930e964.

Message-Id: <19e02f88ab1111d48a60b637f7a9fe3576351481@scylladb.com>

duarten/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla