ALTER TABLE IMPORT doesn't properly handle instant alter metadata.
This patch makes IMPORT read, parse and apply instant alter metadata at the
very beginning of operation. So, cases when source table has some metadata
and destination table doesn't have it now works fine.

DISCARD already removes instant metadata so importing normal table into
instant table worked fine before this patch.

decrypt_decompress(): decrypts and decompresses page if needed

handle_instant_metadata(): this should be the first thing to read source
table. Basically, it applies instant metadata to a destination
dict_table_t object. This is the first thing to read FSP flags so
all possible checks of it were moved to this function.

PageConverter::update_index_page(): it doesn't now read instant metadata.
This logic were moved into handle_instant_metadata()

row_import::match_flags(): this is a first part row_import::match_schema().
As a separate function it's used by handle_instant_metadata().

fil_space_t::is_full_crc32_compressed(): added convenient function

ha_innobase::discard_or_import_tablespace(): do not reload table definition
to read instant metadata because handle_instant_metadata() does it better.
The reverted code was originally added in
4e7ee166

ANONYMOUS_VAR: this is a handy thing to use along with make_scope_exit()

full_crc32_import.test shows different results, because no
dict_table_close() and dict_table_open_on_id() happens.
Thus, SHOW CREATE TABLE shows a little bit older table definition.