ha_cb_send_append_entries_stub's doc-comment now plainly states it is
reachable only via raft_send_append_entries (zero production callers,
grep-confirmed), and names repl_broadcast/ha_stream_wal_to_standby as
the actual production send/retry/backoff mechanism (Req 3.3, amended
to permit this documentation-only resolution over retire-or-wire-in).
Adds an XCE_TESTING-only ha_send_append_entries_stub_call_count
counter, mirroring the ha_wal_ack_send_hook idiom, as the guard against
this ever silently regaining a production caller. Task 4.2 will drive
the real production path and assert it stays zero.
1063 -> 1064 tests, 0 failures. Zero behavior change to any production
send path.
Rongsong Shen
6d798df35d