datetime columns timezone conversion fix

[lpmarques]

As reported here, although mysql datetime values are not stored with any timezone information, Maxwell bootstrap utility adds it to them based on database's timezone, and then proceeds to convert them to client server timezone. Such unexpected behavior does not occur in the CDC process.

This is an attempt on avoiding the problem by treating datetime values as simple strings.

[osheroff]

kinda scary and deep. might work, but would need far more tests around various behaviors.

[lpmarques]

@osheroff, as mentioned in the issue, I've run some tests and confirmed the fix works for datetimes of any precision, making the bootstrapping process coherent with CDC's behavior of replicating datetime values as stored.

What further tests would you suggest?