This commit changes the vtable 'xUpdate' goModule field to a new
'cXUpdate' callback function which in turns calls a 'goVUpdate'
callback.
This new callback allows Go defined virtual table implementations
satisfying the VTabUpdater interface (also newly defined) a way to
delete/insert/update rows in a VTab.
Additionally, an anonymous interface is used within the goVUpdate
callback looking for 'TableName() string' which, when defined on a VTab
is used to provide a better contextual error message to end users if the
VTab is read only.
Care was taken to follow existing code style/conventions for this
addition, and for backwards-compatibility with existing VTab
implementations (hence why a new interface was required).
Kenneth Shaw
Yasuhiro Matsumoto