beefed up sql_step_queue a bit

Summary: basic sql lock implementation

Test Plan: ankur

Reviewers: ankur, anders

Differential Revision: http://grizzly.memsql.com/D3951
2013-10-15 Version 2.6.0

    * Added a useful class called SQLLockManager
        * Produces SQLLock's attached to a database table
        * These provide the means to coordinate resource access in a distributed system
        * Locks timeout if they havn't been pinged in a configurable expiry time
            * Ensure deadlocks can't happen due to parts of the distributed architecture failing