[#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

util/semaphore/semaphore_test.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 )
 
 func TestSemaphore(t *testing.T) {
@@ -24,12 +25,17 @@ func TestSemaphore(t *testing.T) {
 		beforeT, afterT uint32
 	}
 
+	ctx := context.Background()
+	defer trace.WithTaskFromStackUpdateCtx(&ctx)()
+
 	var wg sync.WaitGroup
 	wg.Add(numGoroutines)
 	for i := 0; i < numGoroutines; i++ {
 		go func() {
+			ctx, end := trace.WithTaskFromStack(ctx)
+			defer end()
 			defer wg.Done()
-			res, err := sem.Acquire(context.Background())
+			res, err := sem.Acquire(ctx)
 			require.NoError(t, err)
 			defer res.Release()
 			if time.Since(begin) > sleepTime {