update task_parameter and set value to TEXT type

[jimexist]

fixes #2240

Description

Motivation and Context

Have you tested this? If so, how?

[dlstadther]

I don't have any experience with this portion of Luigi. @erikbern @stephenpascoe could y'all take a look given your past contributions to this module? Thanks!

[jimexist]

thanks for bringing up the conversation. travis failed because i used postgres syntax but the sqlite didn't support.

another thing that i did also want to confirm is that task_id index was not a unique index but it was used like one, i.e. fetching by task_id is assumed to return one row. this was not enforced in database level - but i think it should be.

[kwilcox]

Just recently hit this issue in production when a new task went over the existing 256 character limit. I've tested this PR and migrated an existing postgres database just fine. However, based on some existing issues (#2370), at least one person is using MS SQL and many people use SQLite as their task stores so we at least need to support those dialects. We can just else here https://github.com/Jimexist/luigi/blob/64e861dbfb4f7d851cb3ae6c29e0b54e3dcaf8f5/luigi/db_task_history.py#L261 instead of a else if and cover most of the cases. Oracle will need a specific check since its ALTER TABLE syntax after 10g is a bit different when modifying columns.

[jimexist]

thanks for fixing this