1. Make PyReader and RandomDataGenerator inherited from FileReader.
2. Remove the memeber variable 'dims_' and realated checks in FileReader.

Do not use ctor

* Reduce line of codes.
* We can use virtual function for Maker now.
* The implementation does not care what maker holds, it is easier to
refactor later.

1. Removes MultipleReader's multi-thread support, for we have got
ThreadedReader.
2. Rename MultipleReader to MultiFileReader

* Fluid Channel should match Go Channel in Semantics

* Fix Python channel_send

* Address code rveiew feedback

* Fix open_files_op.cc

* Add description to Channel Asserts