This reverts commit 87fef901.

to definite the reusing external table by themselves instead of
searching in gpload, which may have bad performance if there are
too many external tables.

- Fix gpload.py, add schema prefix to every external table name
  when EXTERNAL.SCHEMA is set
- Add new test cases

1. fix an error if the quote and escape options are not specified in the
CSV format.

2. add codes for delimiter, escape and quote options to process a
ASCII, unicode-encoded or escaped single character in order to find
the correct table to reuse when load external data.

The old cdbfast test suite had these, good to move to ICW.