batchlog_manager: Fix drain() reentrability

Currently drain() is called twise -- first time from
storage_service::drain() (on shutdown), second via
batchlog_manager::stop(). The routine is unintentinally re-entrable,
because:
- explicit check for not aborting the abort source twise
- breaking semaphore can be done multiple times
- co-await-ing of the _started future works because the future is shared

That's not extremely elegant, better to make the drain() bail out early
if it was already called.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

db/batchlog_manager.cc

@@ -115,10 +115,12 @@ future<> db::batchlog_manager::start() {
 }
 
 future<> db::batchlog_manager::drain() {
-    blogger.info("Asked to drain");
-    if (!_stop.abort_requested()) {
-        _stop.request_abort();
+    if (_stop.abort_requested()) {
+        co_return;
     }
+
+    blogger.info("Asked to drain");
+    _stop.request_abort();
     if (this_shard_id() == 0) {
         // Abort do_batch_log_replay if waiting on the semaphore.
         _sem.broken();