mtr_t::page_lock(): Validate the page number.

ibuf_tree_root_get(): Remove assertions that became redundant.

The assertions in btr_validate_level() are kind of redundant as well,
but because they are ut_a(), they are also present in release builds,
while the ones in mtr_t::page_lock() are only present in debug builds.

btr_cur_position(): Do not duplicate an assertion that is part of
page_cur_position().

dict_load_tablespace(): Introduce a new option
DICT_ERR_IGNORE_TABLESPACE that will suppress loading a tablespace
when a table is going to be dropped.