Quota enforcing sector allocator

```
Moves disk quota enforcement to sector allocator, this allows the
file pool to have arbitrary large sparse files without running into
quota issues.

A consequence of this is that very large sparse files are now a first
class citizen of Buildbarn. We have therefore modified the sparse files
implementation to use a struct inspired by the unix inode pointer
struct.
```

Author: meroton-benjamin

cmd/bb_worker/main.go

@@ -76,9 +76,9 @@ func main() {
 		// currently only used by the virtual file system to store
 		// output files of build actions. Going forward, this may be
 		// used to store core dumps generated by build actions as well.
-		filePool, err := pool.NewFilePoolFromConfiguration(configuration.FilePool)
+		filePoolFactory, baseSectorAllocator, sectorSizeBytes, err := pool.NewFilePoolFactoryFromConfiguration(configuration.FilePool)
 		if err != nil {
-			return util.StatusWrap(err, "Failed to create file pool")
+			return util.StatusWrap(err, "Failed to create file pool factory")
 		}
 
 		// Storage access.
@@ -490,13 +490,21 @@ func main() {
 						return util.StatusWrapf(err, "Invalid instance name prefix %#v", runnerConfiguration.InstanceNamePrefix)
 					}
 
+					maximumSectors := runnerConfiguration.MaximumFilePoolSizeBytes / uint64(sectorSizeBytes)
+					runnerFilePool := pool.NewQuotaEnforcingFilePool(
+						filePoolFactory.NewFilePool(
+							pool.NewQuotaEnforcingSectorAllocator(
+								baseSectorAllocator,
+								int64(maximumSectors),
+							),
+						),
+						int64(runnerConfiguration.MaximumFilePoolFileCount),
+					)
+
 					buildClient := builder.NewBuildClient(
 						schedulerClient,
 						buildExecutor,
-						pool.NewQuotaEnforcingFilePool(
-							filePool,
-							runnerConfiguration.MaximumFilePoolFileCount,
-							runnerConfiguration.MaximumFilePoolSizeBytes),
+						runnerFilePool,
 						clock.SystemClock,
 						workerID,
 						instanceNamePrefix,

pkg/filesystem/pool/BUILD.bazel

@@ -11,7 +11,10 @@ go_library(
         "hole_source.go",
         "metrics_file_pool.go",
         "quota_enforcing_file_pool.go",
+        "quota_enforcing_sector_allocator.go",
+        "quota_metric.go",
         "sector_allocator.go",
+        "sector_map.go",
     ],
     importpath = "github.com/buildbarn/bb-remote-execution/pkg/filesystem/pool",
     visibility = ["//visibility:public"],
@@ -33,6 +36,8 @@ go_test(
         "block_device_backed_file_pool_test.go",
         "empty_file_pool_test.go",
         "quota_enforcing_file_pool_test.go",
+        "quota_enforcing_sector_allocator_test.go",
+        "sector_map_test.go",
     ],
     deps = [
         ":pool",