raft: do not add new wait entries after abort
raft: do not add new wait entries after abort

Abort signals stopped_error on all awaited entries, but if an entry is
added after this it will be destroyed without signaling and will cause
a waiter to get broken_promise.

Fixes #9688

Message-Id: <5ebc1307cd85d9c715d6ecc0be933adbf3eaf989@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla