endpoint: refactor, fix stale holds on initial replication, holds release subcmds

- endpoint abstractions now share an interface `Abstraction`
- pkg endpoint now has a query facitilty (`ListAbstractions`) which is
  used to find on-disk
    - step holds and bookmarks
    - replication cursors (v1, v2)
    - last-received-holds
- the `zrepl holds list` command consumes endpoint.ListAbstractions
- the new `zrepl holds release-{all,stale}` commands can be used
  to remove abstractions of package endpoint

Co-authored-by: InsanePrawn <insane.prawny@gmail.com>

supersedes #282
fixes #280
fixes #278

client/holds_list.go

@@ -0,0 +1,71 @@
+package client
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"github.com/fatih/color"
+	"github.com/pkg/errors"
+	"github.com/spf13/pflag"
+	"github.com/zrepl/zrepl/cli"
+	"github.com/zrepl/zrepl/endpoint"
+)
+
+var holdsListFlags struct {
+	Filter holdsFilterFlags
+	Json   bool
+}
+
+var holdsCmdList = &cli.Subcommand{
+	Use:             "list",
+	Run:             doHoldsList,
+	NoRequireConfig: true,
+	Short:           "list holds and bookmarks",
+	SetupFlags: func(f *pflag.FlagSet) {
+		holdsListFlags.Filter.registerHoldsFilterFlags(f, "list")
+		f.BoolVar(&holdsListFlags.Json, "json", false, "emit JSON")
+	},
+}
+
+func doHoldsList(sc *cli.Subcommand, args []string) error {
+	var err error
+	ctx := context.Background()
+
+	if len(args) > 0 {
+		return errors.New("this subcommand takes no positional arguments")
+	}
+
+	q, err := holdsListFlags.Filter.Query()
+	if err != nil {
+		return errors.Wrap(err, "invalid filter specification on command line")
+	}
+
+	abstractions, errors, err := endpoint.ListAbstractions(ctx, q)
+	if err != nil {
+		return err // context clear by invocation of command
+	}
+
+	// always print what we got
+	if holdsListFlags.Json {
+		enc := json.NewEncoder(os.Stdout)
+		enc.SetIndent("", "  ")
+		if err := enc.Encode(abstractions); err != nil {
+			panic(err)
+		}
+		fmt.Println()
+	} else {
+		for _, a := range abstractions {
+			fmt.Println(a)
+		}
+	}
+
+	// then potential errors, so that users always see them
+	if len(errors) > 0 {
+		color.New(color.FgRed).Fprintf(os.Stderr, "there were errors in listing the abstractions:\n%s\n", errors)
+		return fmt.Errorf("")
+	} else {
+		return nil
+	}
+}