- InnoDB DDL results in `Duplicate entry' if concurrent DML throws
duplicate key error. The following scenario explains the problem

connection con1:
  ALTER TABLE t1 FORCE;

connection con2:
  INSERT INTO t1(pk, uk) VALUES (2, 2), (3, 2);

In connection con2, InnoDB throws the 'DUPLICATE KEY' error because
of unique index. Alter operation will throw the error when applying
the concurrent DML log.

- Inserting the duplicate key for unique index logs the insert
operation for online ALTER TABLE. When insertion fails,
transaction does rollback and it leads to logging of
delete operation for online ALTER TABLE.
While applying the insert log entries, alter operation
encounters 'DUPLICATE KEY' error.

- To avoid the above fake duplicate scenario, InnoDB should
not write any log for online ALTER TABLE before DML transaction
commit.

- User thread which does DML can apply the online log if
InnoDB ran out of online log and index is marked as completed.
Set online log error if apply phase encountered any error.
It can also clear all other indexes log, marks the newly
added indexes as corrupted.

- Removed the old online code which was a part of DML operations

commit_inplace_alter_table() : Does apply the online log
for the last batch of secondary index log and does frees
the log for the completed index.

trx_t::apply_online_log: Set to true while writing the undo
log if the modified table has active DDL

trx_t::apply_log(): Apply the DML changes to online DDL tables

dict_table_t::is_active_ddl(): Returns true if the table
has an active DDL

dict_index_t::online_log_make_dummy(): Assign dummy value
for clustered index online log to indicate the secondary
indexes are being rebuild.

dict_index_t::online_log_is_dummy(): Check whether the online
log has dummy value

ha_innobase_inplace_ctx::log_failure(): Handle the apply log
failure for online DDL transaction

row_log_mark_other_online_index_abort(): Clear out all other
online index log after encountering the error during
row_log_apply()

row_log_get_error(): Get the error happened during row_log_apply()

row_log_online_op(): Does apply the online log if index is
completed and ran out of memory. Returns false if apply log fails

UndorecApplier: Introduced a class to maintain the undo log
record, latched undo buffer page, parse the undo log record,
maintain the undo record type, info bits and update vector

UndorecApplier::get_old_rec(): Get the correct version of the
clustered index record that was modified by the current undo
log record

UndorecApplier::clear_undo_rec(): Clear the undo log related
information after applying the undo log record

UndorecApplier::log_update(): Handle the update, delete undo
log and apply it on online indexes

UndorecApplier::log_insert(): Handle the insert undo log
and apply it on online indexes

UndorecApplier::is_same(): Check whether the given roll pointer
is generated by the current undo log record information

trx_t::rollback_low(): Set apply_online_log for the transaction
after partially rollbacked transaction has any active DDL

prepare_inplace_alter_table_dict(): After allocating the online
log, InnoDB does create fulltext common tables. Fulltext index
doesn't allow the index to be online. So removed the dead
code of online log removal

Thanks to Marko Mäkelä for providing the initial prototype and
Matthias Leich for testing the issue patiently.