Problem:
=======
Access to the following variables is protected by LOCK_log.

binlog-transaction-dependency-tracking
binlog-transaction-dependency-history-size

'LOCK_log' is held when these variables are being set or read.
Holding 'LOCK_log' results in following Deadlock scenario.

Analysis:
=========
1) SELECT * FROM performance_schema.session_variables WHERE
VARIABLE_NAME LIKE 'binlog_transaction_dependency_tracking';

The above query acquires a lock on thread data and tries to
read the values of 'binlog_transaction_dependency_tracking'.
Inorder to read this variable 'LOCK_log' is required.

Owns: THD::LOCK_thd_data (acquired in
      PFS_system_variable_cache::do_materialize_all
      ->PFS_variable_cache<Var_type>::get_THD
      -> Find_THD_variable::operator())
Waits for: MYSQL_BIN_LOG::LOCK_log

2) SHOW BINARY LOGS

Above command acquires 'LOCK_log' to read current active
binary log specific information and then goes on to acquire
LOCK_index, so that it can list the rest of binary logs.

Owns: MYSQL_BIN_LOG::LOCK_log (acquired in show_binlogs())
Waits for: MYSQL_BIN_LOG::LOCK_index

3) PURGE LOGS BEFORE date

Above command acquires 'LOCK_index' and reads one log at a
time from index. For each log it tries to identify how many
threads are accessing this log. Inorder to do this it
acquires a lock on global thread list and iterates through
the entire thread list. For each thread it tries to acquire
LOCK_thd_data and verify if the log is being used by the
tread or not. Hence it waits for the lock.

Owns: MYSQL_BIN_LOG::LOCK_index (acquired in
      MYSQL_BIN_LOG::purge_logs_before_date)
Waits for: THD::LOCK_thd_data

Fix:
===
Transaction dependency tracking information is updated
based on 'max_committed_transaction' object contents.
'max_committed_transaction' holds the transaction
sequence_number. This transaction_sequence number is updated
during the flush stage of the commit and it is used to
update the transaction dependency tracking through
'update_max_committed' function call.

'LOCK_log' needs to be held when the active binary log is
being modified. Where as to protect concurrent access to set
or read dependency tracking information a less granular lock
should be sufficient. Hence a new lock named
'LOCK_slave_trans_dep_tracker' has been introduced to
protect concurrent access to transaction dependency tracking
information.