File api changes

Hi Shaun,

I've made the following update to amber api:

- update file.getSize() to cater for if failed to retrieve the file from Doss while having a blob id.

- adding migrationdao for converting the encoding of the value from one data type to another:
    e.g. from STR to INT, or from INT to LNG

Could you please review these changes?

Thanks

Shuang