[#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

client/migrate.go

@@ -48,14 +48,13 @@ var migratePlaceholder0_1Args struct {
 	dryRun bool
 }
 
-func doMigratePlaceholder0_1(sc *cli.Subcommand, args []string) error {
+func doMigratePlaceholder0_1(ctx context.Context, sc *cli.Subcommand, args []string) error {
 	if len(args) != 0 {
 		return fmt.Errorf("migration does not take arguments, got %v", args)
 	}
 
 	cfg := sc.Config()
 
-	ctx := context.Background()
 	allFSS, err := zfs.ZFSListMapping(ctx, zfs.NoFilter())
 	if err != nil {
 		return errors.Wrap(err, "cannot list filesystems")
@@ -124,7 +123,7 @@ var fail = color.New(color.FgRed)
 
 var migrateReplicationCursorSkipSentinel = fmt.Errorf("skipping this filesystem")
 
-func doMigrateReplicationCursor(sc *cli.Subcommand, args []string) error {
+func doMigrateReplicationCursor(ctx context.Context, sc *cli.Subcommand, args []string) error {
 	if len(args) != 0 {
 		return fmt.Errorf("migration does not take arguments, got %v", args)
 	}
@@ -137,8 +136,6 @@ func doMigrateReplicationCursor(sc *cli.Subcommand, args []string) error {
 		return fmt.Errorf("exiting migration after error")
 	}
 
-	ctx := context.Background()
-
 	v1cursorJobs := make([]job.Job, 0, len(cfg.Jobs))
 	for i, j := range cfg.Jobs {
 		if jobs[i].Name() != j.Name() {