The problem was that during DROP TEMPORARY TABLE we tried to acquire
the name lock, though temporary tables belongs to one connection, and
no race is possible.

The solution is to not use table name locking while executing
DROP TEMPORARY TABLE.


mysql-test/r/temp_table.result:
  Add result for bug#21096: locking issue ; temporary table conflicts.
mysql-test/t/temp_table.test:
  Add test case for bug#21096: locking issue ; temporary table conflicts.
sql/sql_table.cc:
  Do not use table name locking while executing DROP TEMPORARY TABLE.