Add support for Union All

[harshit-gangal]

Added supported for union all. At VTGate in memory concatenation of results will happen.
Current Limitation: If there is column type mismatch between different union it will fail than resolving the type.

Closes: #4613

[sougou]

In case you haven't, can you also make sure that you've sanity checked code coverage?

[sougou]

Minor nits.

[sougou]

As a general rule, we should not change the caller's context. Although things will work as expected in this case, it's better to still follow the rule. This means that you have to create a local cancel-able one and use that instead.

[systay]

I think we need the engine primitives to use this new cancelable context. Contexts are like onions - there are layers to them. We are merely adding a cancelable layer to the context the caller provided us with. if you want, we could peel that layer before leaving this method, to restore the original context.

[sougou]

Oh. I didn't realize that you were passing vcursor down to the callees. So, it's ok to leave it as is for now.

[harshit-gangal]

The context created here needs to be sent down to calls from here one. So that once the context is cancelled. The lower layers can also stop processing.
The issue here is the lower layer can receive this new context via vcursor as of now. So, the current way was to replace it.
Once we start passing context separately this would not be an issue.

@systay The current suggestion does not hold. Because if we replace the vcursor context and there is parent branch which has vcursor usage. It will already be updated and using the new context. So peeling off before returning may not guarantee things are in order.

[sougou]

You can merge after resolving conflicts.