buf_page_t::buf_fix_count transition to Atomic_counter.

Replaced srv_fatal_semaphore_wait_threshold juggling with
btr_validate_index_running counter.
Removed last argument of btr_validate_index(): always false.
Simplified away btr_validate_spatial_index().

Fix an inadvertently inverted condition that caused
galera.galera_sst_mariabackup_table_options test failure.

MDEV-18129 Backup fails for encrypted tables: mariabackup: Database page corruption detected at page 1

If an encrypted table is created during backup, then
mariabackup --backup could wrongly fail.

This caused a failure of the test mariabackup.huge_lsn once on buildbot.

This is due to the way how InnoDB creates .ibd files. It would first
write a dummy page 0 with no encryption information. Due to this,
xb_fil_cur_open() could wrongly interpret that the table is not encrypted.
Subsequently, page_is_corrupted() would compare the computed page
checksum to the wrong checksum. (There are both "before" and "after"
checksums for encrypted pages.)

To work around this problem, we introduce a Boolean option
--backup-encrypted that is enabled by default. With this option,
Mariabackup will assume that a nonzero key_version implies that the
page is encrypted. We need this option in order to be able to copy
encrypted tables from MariaDB 10.1 or 10.2, because unencrypted pages
that were originally created before MySQL 5.1.48 could contain nonzero
garbage in the fields that were repurposed for encryption.

Later, MDEV-18128 would clean up the way how .ibd files are created,
to remove the need for this option.

page_is_corrupted(): Add missing const qualifiers, and do not check
space->crypt_data unless --skip-backup-encrypted has been specified.

xb_fil_cur_read(): After a failed page read, output a page dump.

This is a regression caused by
commit 8c43f963
that was part of the MDEV-12112 fixes.

page_is_corrupted(): Never interpret page_no=0 as encrypted.

This is a regression after MDEV-13671.

The bug is related to key part prefix lengths wich are stored in SYS_FIELDS.
Storage format is not obvious and was handled incorrectly which led to data
dictionary corruption.

SYS_FIELDS.POS actually contains prefix length too in case if any key part
has prefix length.

innobase_rename_column_try(): fixed prefixes handling

Tests for prefixed indexes added too.

Closes #1063

Trivial fil_space_t::n_pending_ops transition. Since it is not
obvious which memory barriers are supposed to be issued, seq_cst
memory order was preserved.

buf_pool_t::n_pend_unzip transition to Atomic_counter.

zip_pad_info_t::pad transition to Atomic_counter.

ibuf_t::n_merged_ops and ibuf_t::n_discarded_ops transition to
Atomic_counter.