[#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

replication/driver/replication_stepqueue.go

@@ -2,8 +2,10 @@ package driver
 
 import (
 	"container/heap"
+	"context"
 	"time"
 
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	"github.com/zrepl/zrepl/util/chainlock"
 )
 
@@ -155,7 +157,8 @@ func (q *stepQueue) sendAndWaitForWakeup(ident interface{}, targetDate time.Time
 }
 
 // Wait for the ident with targetDate to be selected to run.
-func (q *stepQueue) WaitReady(ident interface{}, targetDate time.Time) StepCompletedFunc {
+func (q *stepQueue) WaitReady(ctx context.Context, ident interface{}, targetDate time.Time) StepCompletedFunc {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	if targetDate.IsZero() {
 		panic("targetDate of zero is reserved for marking Done")
 	}