[WIP] add and use tracing API as part of package logging

- make `logging.GetLogger(ctx, Subsys)` the authoritative `logger.Logger` factory function
    - the context carries a linked list of injected fields which
      `logging.GetLogger` adds to the logger it returns
- introduce the concept of tasks and spans, also tracked as linked list within ctx
    - [ ] TODO automatic logging of span begins and ends, with a unique
      ID stack that makes it easy to follow a series of log entries in
      concurrent code
    - ability to produce a chrome://tracing-compatible trace file,
      either via an env variable or a `zrepl pprof` subcommand
        - this is not a CPU profile, we already have go pprof for that
        - but it is very useful to visually inspect where the
          replication / snapshotter / pruner spends its time
          ( fixes #307 )

zfs/versions_destroy.go

@@ -38,8 +38,8 @@ func (o *DestroySnapOp) String() string {
 	return fmt.Sprintf("destroy operation %s@%s", o.Filesystem, o.Name)
 }
 
-func ZFSDestroyFilesystemVersions(reqs []*DestroySnapOp) {
-	doDestroy(context.TODO(), reqs, destroyerSingleton)
+func ZFSDestroyFilesystemVersions(ctx context.Context, reqs []*DestroySnapOp) {
+	doDestroy(ctx, reqs, destroyerSingleton)
 }
 
 func setDestroySnapOpErr(b []*DestroySnapOp, err error) {