migrate: detach context from parent context cancelation

[mmatczuk]

Migrate uses Query with Context 1 2.

The problem is that the context may be canceled when migration statement is sent but before it's recorded in gocqlx migration table.

To solve this I suggest we do use xcontext to detach from the context lifecycle and manually check if the context has been canceled before processing next statement.

[mmatczuk]

cc @karol-kokoszka