[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 )

daemon/job/job.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/endpoint"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/zfs"
@@ -14,21 +15,8 @@ import (
 
 type Logger = logger.Logger
 
-type contextKey int
-
-const (
-	contextKeyLog contextKey = iota
-)
-
 func GetLogger(ctx context.Context) Logger {
-	if l, ok := ctx.Value(contextKeyLog).(Logger); ok {
-		return l
-	}
-	return logger.NewNullLogger()
-}
-
-func WithLogger(ctx context.Context, l Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLog, l)
+	return logging.GetLogger(ctx, logging.SubsysJob)
 }
 
 type Job interface {