add task source-id length check

[amyangfei]

Feature Request

Is your feature request related to a problem? Please describe:

The id field in loader/syncer unit checkpoint table is char(32)/varchar(32), as source-id is defined by user without any length restriction, this may lead to error initialize checkpoint: Error 1406: Data too long for column 'id' at row 1

Describe the feature you'd like:

add source-id length check in check-task, and find out whether more restrictions should be added.

[csuzhangxc]

I think we should also do the check in DM-ansible and DM-worker.