[#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/testcmd.go

@@ -39,7 +39,7 @@ var testFilter = &cli.Subcommand{
 	Run: runTestFilterCmd,
 }
 
-func runTestFilterCmd(subcommand *cli.Subcommand, args []string) error {
+func runTestFilterCmd(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 
 	if testFilterArgs.job == "" {
 		return fmt.Errorf("must specify --job flag")
@@ -49,7 +49,6 @@ func runTestFilterCmd(subcommand *cli.Subcommand, args []string) error {
 	}
 
 	conf := subcommand.Config()
-	ctx := context.Background()
 
 	var confFilter config.FilesystemsFilter
 	job, err := conf.Job(testFilterArgs.job)
@@ -137,10 +136,9 @@ var testPlaceholder = &cli.Subcommand{
 	Run: runTestPlaceholder,
 }
 
-func runTestPlaceholder(subcommand *cli.Subcommand, args []string) error {
+func runTestPlaceholder(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 
 	var checkDPs []*zfs.DatasetPath
-	ctx := context.Background()
 
 	// all actions first
 	if testPlaceholderArgs.all {
@@ -197,11 +195,11 @@ var testDecodeResumeToken = &cli.Subcommand{
 	Run: runTestDecodeResumeTokenCmd,
 }
 
-func runTestDecodeResumeTokenCmd(subcommand *cli.Subcommand, args []string) error {
+func runTestDecodeResumeTokenCmd(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 	if testDecodeResumeTokenArgs.token == "" {
 		return fmt.Errorf("token argument must be specified")
 	}
-	token, err := zfs.ParseResumeToken(context.Background(), testDecodeResumeTokenArgs.token)
+	token, err := zfs.ParseResumeToken(ctx, testDecodeResumeTokenArgs.token)
 	if err != nil {
 		return err
 	}