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

platformtest/tests/tests.go

@@ -18,11 +18,6 @@ var Cases = []Case{
 	UndestroyableSnapshotParsing,
 	GetNonexistent,
 	ReplicationCursor,
-	RollupReleaseIncluding,
-	RollupReleaseExcluding,
-	RollupReleaseMostRecentIsBookmarkWithoutSnapshot,
-	RollupReleaseMostRecentIsBookmarkAndSnapshotStillExists,
-	RollupReleaseMostRecentDoesntExist,
 	IdempotentHold,
 	IdempotentBookmark,
 	IdempotentDestroy,