ibuf.size, ibuf.max_size: Changed the type to Atomic_relaxed<ulint>
in order to fix some (not all) race conditions.

ibuf_contract(): Renamed from ibuf_merge_pages(ulint*).

ibuf_merge(), ibuf_merge_all(): Removed.

srv_shutdown(): Invoke log_free_check() and ibuf_contract(). Even though
ibuf_contract() is not writing anything, it will trigger calls of
ibuf_merge_or_delete_for_page(), which will write something. Because
we cannot invoke log_free_check() at that low level, we must invoke
it at the high level.

srv_shutdown_print(): Replaces srv_shutdown_print_master_pending().
Report progress and remaining work every 15 seconds. For the
change buffer merge, the remaining work is indicated by ibuf.size.