Transaction-scoped parameters for CStore methods

Currently, adjusting database parameters cannot be done in any way except globally, directly in the database. However, there are several runtime parameters that would be extremely useful to expose. In particular, allowing modification of the timezone parameter would support localization efforts for those consortia that span multiple time zones.

Another useful capability would be to move management and use of database replicas to the application level, removing the need for a third-party load ballancer. As this is logically at the same level as transaction-scoped database parameters, the same mechanism could be used to pass information about read-only or read-write transactional requirements to the CStore process, facilitating a move to internal load balancing. CStore and derivatives already contain a significant portion of the logic required to make direct use of multiple databases, so this move would be of relatively low effort.