dirty uncompressed blocks.  Move the page_zip_validate() call to
a more appropriate place, buf_LRU_block_remove_hashed_page().

buf_LRU_block_remove_hashed_page(): Allow bpage->oldest_modification != 0
for uncompressed pages if there is a compressed page that will be preserved.
Release the mutexes before calling buf_print() and buf_LRU_print().

page_zip_validate(): Remove the assertion containing the
buf_frame_get_page_zip() call.  We might already be holding buf_pool->mutex.