Snapshots now created concurrently

By default it uses the number of CPUs as concurrency limit and it can be
changed in config:

```yaml
jobs:
  - name: "zroot-to-zdisk"
    snapshotting:
type: "periodic"
concurrency: 1
```

README.md

@@ -590,6 +590,19 @@ pkg install zrepl-dsh2dsh
 
     See also zrepl/zrepl#403
 
+  * Snapshots now created concurrently.
+
+    By default it uses the number of CPUs as concurrency limit and it can be
+    changed in config:
+
+    ```yaml
+    jobs:
+      - name: "zroot-to-zdisk"
+        snapshotting:
+          type: "periodic"
+          concurrency: 1
+    ```
+
 ## Upstream user documentation
 
 **User Documentation** can be found at

internal/config/config.go

@@ -342,6 +342,7 @@ type SnapshottingPeriodic struct {
 	Hooks           []HookCommand `yaml:"hooks" validate:"dive"`
 	TimestampFormat string        `yaml:"timestamp_format" default:"dense" validate:"required"`
 	TimestampLocal  bool          `yaml:"timestamp_local" default:"true"`
+	Concurrency     uint          `yaml:"concurrency"`
 }
 
 func (self *SnapshottingPeriodic) CronSpec() string {

internal/daemon/hooks/hook_config.go

@@ -7,8 +7,6 @@ import (
 	"github.com/dsh2dsh/zrepl/internal/zfs"
 )
 
-type List []*CommandHook
-
 func ListFromConfig(in []config.HookCommand) (List, error) {
 	hl := make(List, len(in))
 	for i, h := range in {
@@ -21,6 +19,10 @@ func ListFromConfig(in []config.HookCommand) (List, error) {
 	return hl, nil
 }
 
+type List []*CommandHook
+
+func (self List) Slice() []*CommandHook { return []*CommandHook(self) }
+
 func (self List) WithCombinedOutput() List {
 	for _, h := range self {
 		h.WithCombinedOutput()