Also, MDEV-14317 When ALTER TABLE is aborted, do not write garbage pages to data files

As pointed out by Shaohua Wang, the merge of MDEV-13328 from
MariaDB 10.1 (based on MySQL 5.6) to 10.2 (based on 5.7)
was performed incorrectly.

Let us always pass a non-NULL FlushObserver* when writing
to data files is desired.

FlushObserver::is_partial_flush(): Check if this is a bulk-load
(partial flush of the tablespace).

FlushObserver::is_interrupted(): Check for interrupt status.

buf_LRU_flush_or_remove_pages(): Instead of trx_t*, take
FlushObserver* as a parameter.

buf_flush_or_remove_pages(): Remove the parameters flush, trx.
If observer!=NULL, write out the data pages. Use the new predicate
observer->is_partial() to distinguish a partial tablespace flush
(after bulk-loading) from a full tablespace flush (export).
Return a bool (whether all pages were removed from the flush_list).

buf_flush_dirty_pages(): Remove the parameter trx.