lsa: Reclaim 1 segment by default

Reclaiming many segments was observed to cause up to multi-ms
latency. With the new setting, the latency of reclamation cycle with
full segments (worst case mode) is below 1ms.

I saw no decrease in throughput compared to the step of 16 segments in
neither of these modes:
  - full segments, reclaim by random evicition
  - sparse segments (3% occupancy), reclaim by compaction and no eviction

Fixes #1274.

utils/logalloc.cc

@@ -55,7 +55,7 @@ class tracker::impl {
     std::vector<region::impl*> _regions;
     scollectd::registrations _collectd_registrations;
     bool _reclaiming_enabled = true;
-    size_t _reclamation_step = 16;
+    size_t _reclamation_step = 1;
 private:
     // Prevents tracker's reclaimer from running while live. Reclaimer may be
     // invoked synchronously with allocator. This guard ensures that this