branches/zip: Fix the decompression of deleted records containing externally stored columns.
branches/zip: Fix the decompression of deleted records containing externally
stored columns.

page_zip_decompress(): We indeed need to clear the BLOB pointers of deleted
records twice.  This implies that we also need to invoke rec_get_offsets()
twice for each record on a leaf page.  Add clarifying comments.

page_zip_clear_rec(): Add the flag page_zip_clear_rec_disable for
disabling the clearing of deleted records.

flynn1973/mariadb-aix,ollie314/server,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,ollie314/server,flynn1973/mariadb-aix,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,ollie314/server,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,ollie314/server,slanterns/server,natsys/mariadb_10.2,natsys/mariadb_10.2