Allow same root_fs for different jobs: sinks and so on (#752)

Because some jobs add client identity to root_fs and other jobs don't do
that,
we can't reliable detect overlapping of filesystems. And and the same
time we
need an ability to use equal or overlapped root_fs for different jobs.
For
instance see this config:

```
  - name: "zdisk"
    type: "sink"
    root_fs: "zdisk/zrepl"
    serve:
      type: "local"
      listener_name: "zdisk"
```
and
```
  - name: "remote-to-zdisk"
    type: "pull"
    connect:
      type: "tls"
    root_fs: "zdisk/zrepl/remote"
```

As you can see, two jobs have overlapped root_fs, but actually datasets
are not
overlapped, because job `zdisk` save everything under
`zdisk/zrepl/localhost`,
because it adds client identity. So they actually use two different
filesystems:
`zdisk/zrepl/localhost` and `zdisk/zrepl/remote`. And we can't detect
this
situation during config check. So let's just remove this check, because
it's
admin's duty to configure correct root_fs's.

---------

Co-authored-by: Christian Schwarz <me@cschwarz.com>

daemon/job/build_jobs.go

@@ -24,21 +24,6 @@ func JobsFromConfig(c *config.Config, parseFlags config.ParseFlags) ([]Job, erro
 		js[i] = j
 	}
 
-	// receiving-side root filesystems must not overlap
-	{
-		rfss := make([]string, 0, len(js))
-		for _, j := range js {
-			jrfs, ok := j.OwnedDatasetSubtreeRoot()
-			if !ok {
-				continue
-			}
-			rfss = append(rfss, jrfs.ToString())
-		}
-		if err := validateReceivingSidesDoNotOverlap(rfss); err != nil {
-			return nil, err
-		}
-	}
-
 	return js, nil
 }
 

docs/configuration/overview.rst

@@ -258,6 +258,9 @@ On your setup, ensure that
 
 * all ``filesystems`` filter specifications are disjoint
 * no ``root_fs`` is a prefix or equal to another ``root_fs``
+
+  * For ``sink`` jobs, consider all possible ``root_fs/${client_identity}``.
+
 * no ``filesystems`` filter matches any ``root_fs``
 
 **Exceptions to the rule**: