Allow registration of custom SqlDialects

This permits using an odd driver type whose URL isn't standard,
a subclass of SqlDialect can be created and registered, making
it available to any Database instance.

Change-Id: Ia04ee41154161be73c28f35783ad29e9cfc78f46
Signed-off-by: Shawn O. Pearce <sop@google.com>
6 files changed