Define the merge_rec_ structures in row0merge.c instead of
row0merge.h.  They are not used in the interface defined in row0merge.h.

row_merge_create_file_structure(): Rename to row_merge_file_create().
Input a pointer to a pre-allocated block of memory.

row_merge_read_clustered_index(): Replace merge_file_t** with merge_file_t*.

row_merge_sort_and_insert_thread(): Remove the declaration.
This function was never defined or called.

row_merge_is_index_usable(): Correct the formatting.

row_build_index_for_mysql(): Allocate an array of merge_file_t instead
of an array of pointers to merge_file_t.