MDEV-23855: Implement asynchronous doublewrite

Synchronous writes and calls to fdatasync(), fsync() or
FlushFileBuffers() would ruin performance. So, let us
submit asynchronous writes for the doublewrite buffer.
We submit a single request for the likely case that the
two doublewrite buffers are contiquous in the system tablespace.

buf_dblwr_t::flush_buffered_writes_completed(): The completion callback
of buf_dblwr_t::flush_buffered_writes().

os_aio_wait_until_no_pending_writes(): Also wait for doublewrite batches.

buf_dblwr_t::element::space: Remove. We can simply use
element::request.node->space instead.

Reviewed by: Vladislav Vaintroub

storage/innobase/include/os0file.h

@@ -198,6 +198,8 @@ public:
     WRITE_SYNC= 16,
     /** Asynchronous write */
     WRITE_ASYNC= WRITE_SYNC | 1,
+    /** A doublewrite batch */
+    DBLWR_BATCH= WRITE_ASYNC | 8,
     /** Write data; evict the block on write completion */
     WRITE_LRU= WRITE_ASYNC | 32,
     /** Write data and punch hole for the rest */