The connection of dashboard to TiDB should not be affected by some global variables

[jackysp]

Bug Report

Please answer these questions before submitting your issue. Thanks!

What did you do?
Some clusters will set global variables like max_execution_time to make the database could fail fast queries.

What did you expect to see?
Some of these variables should not affect dashboard.

What did you see instead?
The slow query page failed to execute, because it exceeded the max_execution_time.

What version of TiDB Dashboard are you using (./tidb-dashboard --version)?
v7.1.1

[jackysp]

PTAL @baurine @mornyx

[jackysp]

It's better to set max_execution_time to 0 when creating the new database session in tidb-dashboard.

[mornyx]

Submitted a PR: #1608.

Setting max_execution_time to 0 can be risky (especially if it has been set to a lower value by the customer). So I set it to 600s. Please also help review this value, thanks. cc @baurine