Updated --help text to declare --slave-parallel-threads as an alpha feature

mysql-test/r/mysqld--help.result:
  Updated --help text
sql/slave.cc:
  Added missing trans_retries++ that caused rpl_deadlock_innodb.test to fail.
  This is safe as this part is never run in parallel.
sql/sql_base.cc:
  Fixed temporary table handling (part of merge)
sql/sys_vars.cc:
  Updated --help text to declare --slave-parallel-threads as an alpha feature