[#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

platformtest/harness/harness.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/fatih/color"
 	"github.com/pkg/errors"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/logging"
@@ -68,7 +69,9 @@ func doMain() error {
 		logger.Error(err.Error())
 		panic(err)
 	}
-	ctx := platformtest.WithLogger(context.Background(), logger)
+	ctx := context.Background()
+	defer trace.WithTaskFromStackUpdateCtx(&ctx)()
+	ctx = logging.WithLoggers(ctx, logging.SubsystemLoggersWithUniversalLogger(logger))
 	ex := platformtest.NewEx(logger)
 
 	type invocation struct {

platformtest/platformtest_logging.go

@@ -3,22 +3,12 @@ package platformtest
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 )
 
 type Logger = logger.Logger
 
-type contextKey int
-
-const (
-	contextKeyLogger contextKey = iota
-)
-
-func WithLogger(ctx context.Context, logger Logger) context.Context {
-	ctx = context.WithValue(ctx, contextKeyLogger, logger)
-	return ctx
-}
-
 func GetLog(ctx context.Context) Logger {
-	return ctx.Value(contextKeyLogger).(Logger)
+	return logging.GetLogger(ctx, logging.SubsysPlatformtest)
 }

platformtest/tests/batchDestroy.go

@@ -32,7 +32,7 @@ func BatchDestroy(ctx *platformtest.Context) {
 			Name:       "2",
 		},
 	}
-	zfs.ZFSDestroyFilesystemVersions(reqs)
+	zfs.ZFSDestroyFilesystemVersions(ctx, reqs)
 	if *reqs[0].ErrOut != nil {
 		panic("expecting no error")
 	}