[#307] add package trace, integrate it with logging, and adopt it throughout zrepl

package trace:

- introduce the concept of tasks and spans, tracked as linked list within ctx
    - see package-level docs for an overview of the concepts
    - **main feature 1**: unique stack of task and span IDs
        - makes it easy to follow a series of log entries in concurrent code
    - **main feature 2**: 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 )

usage in package daemon/logging:

- goal: every log entry should have a trace field with the ID stack from package trace

- 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
    - `logging.GetLogger` also uses package `trace` to get the
      task-and-span-stack and injects it into the returned logger's fields

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) {