For more convenient monitoring of something that could greatly affect
the volume of page writes, we add the status variable
Innodb_buffer_pool_pages_split that was previously only available
via information_schema.innodb_metrics as "innodb_page_splits".
This was suggested by Axel Schwenke.

buf_flush_page_count: Replaced with buf_pool.stat.n_pages_written.
We protect buf_pool.stat (except n_page_gets) with buf_pool.mutex
and remove unnecessary export_vars indirection.

buf_pool.flush_list_bytes: Moved from buf_pool.stat.flush_list_bytes.
Protected by buf_pool.flush_list_mutex.

buf_pool_t::page_cleaner_status: Replaces buf_pool_t::n_flush_LRU_,
buf_pool_t::n_flush_list_, and buf_pool_t::page_cleaner_is_idle.
Protected by buf_pool.flush_list_mutex. We will exclusively broadcast
buf_pool.done_flush_list by the buf_flush_page_cleaner thread,
and only wait for it when communicating with buf_flush_page_cleaner.
There is no need to keep a count of pending writes by the
buf_pool.flush_list processing. A single flag suffices for that.

Waits for page write completion can be performed by
simply waiting on block->page.lock, or by invoking
buf_dblwr.wait_for_page_writes().

buf_LRU_block_free_non_file_page(): Broadcast buf_pool.done_free and
set buf_pool.try_LRU_scan when freeing a page. This would be
executed also as part of buf_page_write_complete().

buf_page_write_complete(): Do not broadcast buf_pool.done_flush_list,
and do not acquire buf_pool.mutex unless buf_pool.LRU eviction is needed.
Let buf_dblwr count all writes to persistent pages and broadcast a
condition variable when no outstanding writes remain.

buf_flush_page_cleaner(): Prioritize LRU flushing and eviction right after
"furious flushing" (lsn_limit). Simplify the conditions and reduce the
hold time of buf_pool.flush_list_mutex. Refuse to shut down
or sleep if buf_pool.ran_out(), that is, LRU eviction is needed.

buf_pool_t::page_cleaner_wakeup(): Add the optional parameter for_LRU.

buf_LRU_get_free_block(): Protect buf_lru_free_blocks_error_printed
with buf_pool.mutex. Invoke buf_pool.page_cleaner_wakeup(true) to
to ensure that buf_flush_page_cleaner() will process the LRU flush
request.

buf_do_LRU_batch(), buf_flush_list(), buf_flush_list_space():
Update buf_pool.stat.n_pages_written when submitting writes
(while holding buf_pool.mutex), not when completing them.

buf_page_t::flush(), buf_flush_discard_page(): Require that
the page U-latch be acquired upfront, and remove
buf_page_t::ready_for_flush().

buf_pool_t::delete_from_flush_list(): Remove the parameter "bool clear".

buf_flush_page(): Count pending page writes via buf_dblwr.

buf_flush_try_neighbors(): Take the block of page_id as a parameter.
If the tablespace is dropped before our page has been written out,
release the page U-latch.

buf_pool_invalidate(): Let the caller ensure that there are no
outstanding writes.

buf_flush_wait_batch_end(false),
buf_flush_wait_batch_end_acquiring_mutex(false):
Replaced with buf_dblwr.wait_for_page_writes().

buf_flush_wait_LRU_batch_end(): Replaces buf_flush_wait_batch_end(true).

buf_flush_list(): Remove some broadcast of buf_pool.done_flush_list.

buf_flush_buffer_pool(): Invoke also buf_dblwr.wait_for_page_writes().

buf_pool_t::io_pending(), buf_pool_t::n_flush_list(): Remove.
Outstanding writes are reflected by buf_dblwr.pending_writes().

buf_dblwr_t::init(): New function, to initialize the mutex and
the condition variables, but not the backing store.

buf_dblwr_t::is_created(): Replaces buf_dblwr_t::is_initialised().

buf_dblwr_t::pending_writes(), buf_dblwr_t::writes_pending:
Keeps track of writes of persistent data pages.

buf_flush_LRU(): Allow calls while LRU flushing may be in progress
in another thread.

Tested by Matthias Leich (correctness) and Axel Schwenke (performance)