sync-diff-inspector: use tidb snapshot reading mechanism when "snapshot" is not set

[tshqin]

Feature Request

Is your feature request related to a problem? Please describe:
no

Describe the feature you'd like:
currently sync-diff-inspector offers non locking read when "snapshot" is not set, this is dangerous cause the data being compared is not ACID consistent. To ensure the consistent read, we need to use tidb snapshot reading mechanism even when "snapshot" is not set.