[#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

rpc/dataconn/stream/stream.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"net"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"unicode/utf8"
 
@@ -80,9 +81,13 @@ func doWriteStream(ctx context.Context, c *heartbeatconn.Conn, stream io.Reader,
 		err error
 	}
 
+	var wg sync.WaitGroup
+	defer wg.Wait()
 	reads := make(chan read, 5)
 	var stopReading uint32
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		defer close(reads)
 		for atomic.LoadUint32(&stopReading) == 0 {
 			buffer := bufpool.Get(1 << FramePayloadShift)