[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/zfs_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// FIXME make this a platformtest
 func TestZFSListHandlesProducesZFSErrorOnNonZeroExit(t *testing.T) {
 	t.SkipNow() // FIXME ZFS_BINARY does not work if tests run in parallel