Simplify instrumentation for LRU eviction

Let us add the status variable innodb_buffer_pool_pages_LRU_freed to monitor the number of pages that were freed by a buffer pool LRU eviction scan, without flushing.

Also, let us simplify the information_schema.innodb_metrics interface (which we hope to eventually remove in MDEV-15706) by removing the following:

• buffer_LRU_batches_flush
• buffer_LRU_batch_flush_pages
• buffer_LRU_batches_evict
• buffer_LRU_batch_evict_pages

and by making the following reflect the status variables:

• buffer_LRU_batch_flush_total_pages: innodb_buffer_pool_pages_LRU_flushed
• buffer_LRU_batch_evict_total_pages: innodb_buffer_pool_pages_LRU_freed