[#316] endpoint / replication protocol: more robust step-holds and replication cursor management

- drop HintMostRecentCommonAncestor rpc call
    - it is wrong to put faith into the active side of the replication to always make that call
      (we might not trust it, ref pull setup)
- clean up step holds + step bookmarks + replication cursor bookmarks on
  send RPC instead
    - this makes it symmetric with Receive RPC
- use a cache (endpoint.sendAbstractionsCache) to avoid the cost of
  listing the on-disk endpoint abstractions state on every step

The "create" methods for endpoint abstractions (CreateReplicationCursor, HoldStep) are now fully
idempotent and return an Abstraction.

Notes about endpoint.sendAbstractionsCache:
- fills lazily from disk state on first `Get` operation
- fill from disk is generally only attempted once
    - unless the `ListAbstractions` fails, in which case the fill from
      disk is retried on next `Get` (the current `Get` will observe a
      subset of the actual on-disk abstractions)
    - the `Invalidate` method is called
- it is a global (zrepl process-wide) cache

fixes #316

platformtest/tests/idempotentBookmark.go

@@ -3,6 +3,7 @@ package tests
 import (
 	"fmt"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/zrepl/zrepl/platformtest"
 	"github.com/zrepl/zrepl/zfs"
 )
@@ -19,22 +20,23 @@ func IdempotentBookmark(ctx *platformtest.Context) {
 
 	fs := fmt.Sprintf("%s/foo bar", ctx.RootDataset)
 
-	asnap := sendArgVersion(ctx, fs, "@a snap")
-	anotherSnap := sendArgVersion(ctx, fs, "@another snap")
+	asnap := fsversion(ctx, fs, "@a snap")
+	anotherSnap := fsversion(ctx, fs, "@another snap")
 
-	err := zfs.ZFSBookmark(ctx, fs, asnap, "a bookmark")
+	aBookmark, err := zfs.ZFSBookmark(ctx, fs, asnap, "a bookmark")
 	if err != nil {
 		panic(err)
 	}
 
 	// do it again, should be idempotent
-	err = zfs.ZFSBookmark(ctx, fs, asnap, "a bookmark")
+	aBookmarkIdemp, err := zfs.ZFSBookmark(ctx, fs, asnap, "a bookmark")
 	if err != nil {
 		panic(err)
 	}
+	assert.Equal(ctx, aBookmark, aBookmarkIdemp)
 
 	// should fail for another snapshot
-	err = zfs.ZFSBookmark(ctx, fs, anotherSnap, "a bookmark")
+	_, err = zfs.ZFSBookmark(ctx, fs, anotherSnap, "a bookmark")
 	if err == nil {
 		panic(err)
 	}
@@ -48,7 +50,7 @@ func IdempotentBookmark(ctx *platformtest.Context) {
 	}
 
 	// do it again, should fail with special error type
-	err = zfs.ZFSBookmark(ctx, fs, asnap, "a bookmark")
+	_, err = zfs.ZFSBookmark(ctx, fs, asnap, "a bookmark")
 	if err == nil {
 		panic(err)
 	}