[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 )

zfs/zfscmd/zfscmd-logging-scraper/zfscmd_logging_scraper.go

@@ -23,7 +23,7 @@ type RuntimeLine struct {
 	Error                        string
 }
 
-var humanFormatterLineRE = regexp.MustCompile(`^(\[[^\]]+\]){2}\[zfs.cmd\]:\s+command\s+exited\s+(with|without)\s+error\s+(.+)`)
+var humanFormatterLineRE = regexp.MustCompile(`^(\[[^\]]+\]){2}\[zfs.cmd\]\[[^\]]+\]:\s+command\s+exited\s+(with|without)\s+error\s+(.+)`)
 
 func parseSecs(s string) (time.Duration, error) {
 	d, err := time.ParseDuration(s + "s")