[MySQL][Spider]Spider-2.21 released

I'm pleased to announce the release of Spider storage engine version 2.21(beta).
Spider is a Storage Engine for database sharding.

The main changes in this version are following.
- Add table parameter "bka_mode" and "bka_engine".
- Add server parameter "spider_bka_mode" and "spider_bka_engine".
  This parameters are for improvement performance for using BKA and using a lot of conditions with IN clause and so on.

"mysql-5.1.44.slave-trx-retry.diff" is included in the source tar ball from this release.
mysql-5.1.44.slave-trx-retry.diff adds a option "slave_transaction_retry_errors". This parameter is used like "slave_transaction_retry_errors=1158,1159,2013,12701". Slave sql thread retries the transaction if slave sql thread encounters the errors at this parameter. Number of retries are at "slave_transaction_retries".

Please see "99_change_logs.txt" in the download documents for checking other changes.
Thanks to Mikiya for your advice.