SQLSRV support for CI Sessions

According to the documentation 

Only MySQL and PostgreSQL databases are officially supported, due to lack of advisory locking mechanisms on other platforms

However MSSQL does support such mechanisms by using sp_getapplock and sp_releaseapplock which also support timeouts, while only requiring public role membership.

Please consider supporting MSSQL for sessions, I believe that this will be beneficial for all who develop applications on the Codeigniter framework in conjunction with a MSSQL database.