if auth_root_socket is set, root user should be using unix_socket

Followup for 4abb8216

Revert the side effect of 7c40996c.
Do not convert password hash to its binary representation when a user
entry is loaded. Do it lazily on the first authenticatation attempt.

As a collateral - force all authentication plugins to follow the
protocol and read_packet at least once before accessing info->username
(username is not available before first client handshake packet is read).

Fix PAM and GSSAPI plugins to behave.

add tests

Add a system variable spider_slave_trx_isolation.
- spider_slave_trx_isolation
  The transaction isolation level when Spider table is used by slave SQL thread.
  -1 : OFF
   0 : READ UNCOMMITTED
   1 : READ COMMITTED
   2 : REPEATABLE READ
   3 : SERIALIZABLE
  The default value is -1

Miscellaneous Spider typos

Change default value of the followings
quick_mode 0 -> 3
quick_page_size 100 -> 1024

Add the following parameter for limiting result page size by byte
- quick_page_byte(qpb)
  Number of bytes in a page when acquisition one by one.
  When quick_mode is 1 or 2, Spider stores at least 1 record even if
  quick_page_byte is smaller than 1 record. When quick_mode is 3,
  quick_page_byte is used for judging using temporary table.
  That is given to priority when server parameter spider_quick_page_byte
  is set.
  The default value is 10485760

Fix "out of sync" issue at using quick_mode = 1 or 2

The fields of the temporary table were not created in create_tmp_table function. Because item->const_item() was true. But the temporary tables that is created by Spider are always used all columns. So Spider should call create_tmp_table function with TMP_TABLE_ALL_COLUMNS flag.

The default value for eq_range_index_dive_limit is set to 10

Replaced wait on COND_thread_count with busy waiting with 1 millisecond
sleep.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.

Signal handler is now respoinsible for setting abort_loop and breaking
poll() in main thread. The rest is handled by main thread itself.

Removed redundant LOCK_error_log init/destroy wrappers.
Removed redundant unireg_end(): it is trivial and it has only one caller.
Removed unused ready_to_exit from PFS.
Removed kill_in_progress: duplicates abort_loop.
Removed shutdown_in_progress: duplicates abort_loop.
Removed ready_to_exit: was used to make sure main thread waits for
cleanups, which are now done by main thread itself.
Removed SIGNALS_DONT_BREAK_READ, MAYBE_BROKEN_SYSCALL,
kill_broken_server: never defined/used.
Make clean_up() static.

fill_status.

Also, remove LOCK_status around calc_sum_of_all_status()

Also, rename LOCK_show_status into LOCK_all_status_vars.
This reflects the variable the lock protects.

modifications (insert/erase) are protected by write lock
iteration over list is protected by read lock.

This way, threads that iterate over the list (as in SHOW PROCESSLIST,
SHOW GLOBAL STATUS) do not block each other.

* fix CRL tests to work
* regenerate certificates to be at least 2048 bit
  (fixes buster and rhel8 in buildbot)
* update generate-ssl-cert.sh to generate crl files
* make all SSL tests to use certificates generated
  in generate-ssl-cert.sh, remove unused certificates

from info to debug

In contrast to thread_count, which is decremented by THD destructor,
this one was most probably intended to be decremented after all THD
destructors are done.

THD_count class was added to achieve similar effect with thread_count.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.
Part of MDEV-15135.

Implemented and integrated THD_list as a replacement for the global
thread list. It uses own mutex instead of LOCK_thread_count for THD
list protection.

Removed unused first_global_thread() and next_global_thread().

delayed_insert_threads is now protected by LOCK_delayed_insert. Although
this patch doesn't fix very wrong synchronization of this variable.

After this patch there are only 2 legitimate uses of LOCK_thread_count
left, both in mysqld.cc: thread_count and ready_to_exit.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.
Part of MDEV-15135.

LOG_INFO::lock was useless. It could've only protect against concurrent
iterators execution, which was already protected by LOCK_thread_count.

Use LOCK_thd_data instead of LOCK_thread_count as a protection against
THD::current_linfo reset.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.
Part of MDEV-15135.

Bootstrap in a separate thread was introduced in 746f0b3b to workaround
OS/2 small stack size. OS/2 support was discontinued in 2006 and modern
operating systems have default stack size a few times larger than
default thread_stack and it is tunable.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.
Part of MDEV-15135.

if ssl_cipher is not specified, it must be NULL, not ""

MDEV-18122 Assertion 'table->versioned() == m_prebuilt->table->versioned()' failed in ha_innobase::open

Closes #1134

and gitignore myrocks_hotbackup (as it's now generated)

Closes #1081

Closes #1080

It's too late. Might work or not (and on buster it doesn't).