Fix method redefinition warnings in threads on Perl >= 5.16

$drh gets cleared on CLONE, but the driver methods persist.
Due to a bug, this didn't warn until Perl 5.16

$ perl -Mthreads -MDBI -MDBD::mysql -we 'DBI->setup_driver("DBD::mysql");' \
  -e 'DBD::mysql->driver; threads->create(sub { DBD::mysql->driver })->join'
Subroutine DBI::db::mysql_fd redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::db::mysql_async_result redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::db::mysql_async_ready redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::st::mysql_async_result redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::st::mysql_async_ready redefined at ${archlib}/DBI.pm line 1393.
Merge pull request #21 from ilmari/thread-driver-warning

Fix method redefinition warnings in threads on Perl >= 5.16