[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/hooks/hooks_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/hooks"
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/zfs"
 )
@@ -69,6 +70,9 @@ func curry(f comparisonAssertionFunc, expected interface{}, right bool) (ret val
 }
 
 func TestHooks(t *testing.T) {
+	ctx, end := logging.WithTaskFromStack(context.Background())
+	defer end()
+
 	testFSName := "testpool/testdataset"
 	testSnapshotName := "testsnap"
 
@@ -418,9 +422,8 @@ jobs:
 
 			cbReached = false
 
-			ctx := context.Background()
 			if testing.Verbose() && !tt.SuppressOutput {
-				ctx = hooks.WithLogger(ctx, log)
+				ctx = logging.WithLoggers(ctx, logging.SubsystemLoggersWithUniversalLogger(log))
 			}
 			plan.Run(ctx, false)
 			report := plan.Report()