MDEV-16329 [5/5] ALTER ONLINE TABLE

* Log rows in online_alter_binlog.
* Table online data is replicated within dedicated binlog file
* Cached data is written on commit.
* Versioning is fully supported.
* Works both wit and without binlog enabled.

* For now savepoints setup is forbidden while ONLINE ALTER goes on.
  Extra support is required. We can simply log the SAVEPOINT query events
  and replicate them together with row events. But it's not implemented
  for now.

* Cache flipping:

  We want to care for the possible bottleneck in the online alter binlog
  reading/writing in advance.

  IO_CACHE does not provide anything better that sequential access,
  besides, only a single write is mutex-protected, which is not suitable,
  since we should write a transaction atomically.

  To solve this, a special layer on top Event_log is implemented.
  There are two IO_CACHE files underneath: one for reading, and one for
  writing.

  Once the read cache is empty, an exclusive lock is acquired (we can wait
  for a currently active transaction finish writing), and flip() is emitted,
  i.e. the write cache is reopened for read, and the read cache is emptied,
  and reopened for writing.

  This reminds a buffer flip that happens in accelerated graphics
  (DirectX/OpenGL/etc).

  Cache_flip_event_log is considered non-blocking for a single reader and a
  single writer in this sense, with the only lock held by reader during flip.

  An alternative approach by implementing a fair concurrent circular buffer
  is described in MDEV-24676.

* Cache managers:
  We have two cache sinks: statement and transactional.
  It is important that the changes are first cached per-statement and
  per-transaction.
  If a statement fails, then only statement data is rolled back. The
  transaction moves along, however.

  Turns out, there's no guarantee that TABLE well persist in
  thd->open_tables to the transaction commit moment.
  If an error occurs, tables from statement are purged.
  Therefore, we can't store te caches in TABLE. Ideally, it should be
  handlerton, but we cut the corner and store it in THD in a list.

mysql-test/main/mdl_sync.test

@@ -1196,14 +1196,14 @@ insert into t1 values (1);
 connection default;
 --echo # Add pending SNW lock.
 --echo # Sending:
---send alter table t1 add primary key (c1);
+--send alter table t1 add primary key (c1), lock=shared;
 --echo # 
 connection mdl_con1;
 --echo # Check that ALTER TABLE is waiting with pending SNW lock.
 let $wait_condition=
 select count(*) = 1 from information_schema.processlist
 where state = "Waiting for table metadata lock" and
-      info = "alter table t1 add primary key (c1)";
+      info = "alter table t1 add primary key (c1), lock=shared";
 --source include/wait_condition.inc
 --echo # Check that S, SH and SR locks are compatible with pending SNW
 handler t1 open t;
@@ -2431,7 +2431,7 @@ create table t1 (i int);
 --echo # Ensure that ALTER waits once it has acquired SNW lock.
 set debug_sync='alter_table_copy_after_lock_upgrade SIGNAL parked1 WAIT_FOR go1';
 --echo # Sending:
---send alter table t1 add column j int
+--send alter table t1 add column j int, lock=shared
 
 --echo #
 connection deadlock_con1;
@@ -2986,7 +2986,7 @@ select * from t1 where c2 = 3;
 --echo #
 connection con46273;
 set debug_sync='alter_table_copy_after_lock_upgrade SIGNAL alter_table_locked WAIT_FOR alter_go';
---send alter table t1 add column e int, rename to t2;
+--send alter table t1 add column e int, rename to t2, lock=shared;
 
 --echo #
 connection default;
@@ -3333,7 +3333,7 @@ connection default;
 --echo # table lock and get blocked on sync point.
 set debug_sync= 'alter_table_copy_after_lock_upgrade SIGNAL parked WAIT_FOR go';
 --echo # Sending:
---send alter table t1 add column j int
+--send alter table t1 add column j int, lock=shared
 
 connection con1;
 --echo # Wait until ALTER TABLE gets blocked on a sync point.