The record MLOG_INDEX_LOAD is supposed to be written to indicate that
some page modifications bypassed redo logging, and that redo logging
is now re-enabled. It was not written for fulltext indexes during
ALTER TABLE.

row_merge_write_redo(): Declare globally. Assert that the index
is neither a spatial nor fulltext index.

recv_mlog_index_load(): Observe a MLOG_INDEX_LOAD operation.

recv_parse_log_recs(): Handle MLOG_INDEX_LOAD also in multi-record
mini-transactions. Because of this omission, we should keep writing
MLOG_INDEX_LOAD in single-record mini-transactions, because older
versions of Mariabackup would fail.

row_fts_merge_insert(): Write MLOG_INDEX_LOAD for the auxiliary
tables of fulltext indexes.