What problem does this PR solve?

Issue Number: close #309

What is changed and how it works?

When running these codes, the ctx has no value WRITER_CONTEXT, so ctx.Value(WRITER_CONTEXT) is nil. It can be fixed by adding ctx value before executing explain analyze statement.
Also, we don't want to really write data when executing explain analyze select into, so we can define a dummy writer, which will write the data into io.Discard.

How Has This Been Tested?

I insert some data into db0..cpu_load, and try explain analyze select * into db1..cpuload from db0..cpu_load, it works properly:




Also I add integretion test:

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules