Change default of float_precision for read_csv and read_table to "high"
- [x] closes #17154
- [x] tests added / passed
   - modified `tests/io/parser/test_c_parser.py` to make sure all 4 options are tested
   - added `tests/io/parser/test_c_parser.py:test_high_is_default`
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
  - for version 1.2

See discussion at bottom of #36149 for the performance tests.  Added `float_precision="legacy"` so people can pick up the old parser.  Can't change default to "high" because of incompatibility with python parser

Hello @Dr-Irv! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-09-11 22:09:32 UTC If there is no performance difference should we not just get rid of the legacy parsing altogether? > If there is no performance difference should we not just get rid of the legacy parsing altogether?

My only concern here is that maybe someone has code that inadvertently depends on it, so we have to keep it in there for some form of compatibility.  I do think we could deprecate the legacy parsing
 yep that sounds fine to leave th option

just update the doc string to indicate  @jreback added the check for an invalid `float_precision` option and now all green