Add table restriction check to changefeed

[amyangfei]

Feature Request

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

TiCDC can only replicate table that meets the following restrictions

• either have a PRIMARY KEY
• or have a UNIQUE KEY and each column in the UNIQUE KEY is declared with NOT NULL

Describe the feature you'd like:

• We should provide a library to check whether a table meets the restriction.
• And when a user creates a new changefeed, show warnings for each table that the changefeed should replicate it, but doesn't meet the reaplication restricion.

[amyangfei]

solved in #368