New INFO field aof_delayed_fsync introduced.

This new field counts all the times Redis is configured with AOF enabled and
fsync policy 'everysec', but the previous fsync performed by the
background thread was not able to complete within two seconds, forcing
Redis to perform a write against the AOF file while the fsync is still
in progress (likely a blocking operation).

src/redis.h

@@ -638,6 +638,7 @@ struct redisServer {
     int aof_selected_db; /* Currently selected DB in AOF */
     time_t aof_flush_postponed_start; /* UNIX time of postponed AOF flush */
     time_t aof_last_fsync;            /* UNIX time of last fsync() */
+    unsigned long aof_delayed_fsync;  /* delayed AOF fsync() counter */
     /* RDB persistence */
     long long dirty;                /* Changes to DB from the last save */
     long long dirty_before_bgsave;  /* Used to restore dirty on failed BGSAVE */