pageserver: simplify LayerAccessStats (#8431)

## Problem

LayerAccessStats contains a lot of detail that we don't use: short
histories of most recent accesses, specifics on what kind of task
accessed a layer, etc. This is all stored inside a Mutex, which is
locked every time something accesses a layer.

## Summary of changes

- Store timestamps at a very low resolution (to the nearest second),
sufficient for use on the timescales of eviction.
- Pack access time and last residence change time into a single u64
- Use the high bits of the u64 for other flags, including the new layer
visibility concept.
- Simplify the external-facing model for access stats to just include
what we now track.

Note that the `HistoryBufferWithDropCounter` is removed here because it
is no longer used. I do not dislike this type, we just happen not to use
it for anything else at present.


Co-authored-by: Christian Schwarz <christian@neon.tech>

test_runner/regress/test_disk_usage_eviction.py

@@ -21,6 +21,10 @@ from fixtures.utils import human_bytes, wait_until
 
 GLOBAL_LRU_LOG_LINE = "tenant_min_resident_size-respecting LRU would not relieve pressure, evicting more following global LRU policy"
 
+# access times in the pageserver are stored at a very low resolution: to generate meaningfully different
+# values, tests must inject sleeps
+ATIME_RESOLUTION = 2
+
 
 @pytest.mark.parametrize("config_level_override", [None, 400])
 def test_min_resident_size_override_handling(
@@ -546,6 +550,7 @@ def test_partial_evict_tenant(eviction_env: EvictionEnv, order: EvictionOrder):
     (tenant_id, timeline_id) = warm
 
     # make picked tenant more recently used than the other one
+    time.sleep(ATIME_RESOLUTION)
     env.warm_up_tenant(tenant_id)
 
     # Build up enough pressure to require evictions from both tenants,
@@ -622,6 +627,10 @@ def test_fast_growing_tenant(neon_env_builder: NeonEnvBuilder, pg_bin: PgBin, or
     for scale in [1, 1, 1, 4]:
         timelines.append((pgbench_init_tenant(layer_size, scale, env, pg_bin), scale))
 
+        # Eviction times are stored at a low resolution.  We must ensure that the time between
+        # tenants is long enough for the pageserver to distinguish them.
+        time.sleep(ATIME_RESOLUTION)
+
     env.neon_cli.safekeeper_stop()
 
     for (tenant_id, timeline_id), scale in timelines:

test_runner/regress/test_threshold_based_eviction.py

@@ -52,8 +52,8 @@ def test_threshold_based_eviction(
         "kind": "NoEviction"
     }
 
-    eviction_threshold = 5
-    eviction_period = 1
+    eviction_threshold = 10
+    eviction_period = 2
     ps_http.set_tenant_config(
         tenant_id,
         {