[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 )

replication/driver/replication_driver_test.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/replication/report"
 
 	"github.com/stretchr/testify/assert"
@@ -149,6 +150,7 @@ func (f *mockStep) ReportInfo() *report.StepInfo {
 func TestReplication(t *testing.T) {
 
 	ctx := context.Background()
+	defer logging.WithTaskFromStackUpdateCtx(&ctx)()
 
 	mp := &mockPlanner{}
 	getReport, wait := Do(ctx, mp)