BUG: Conflict between thousands sep and date parser.

Fixes issue where thousands separator could conflict with date
parsing.

This is only fixed in the C parser.

Closes issue #4678
BUG: fix issue #4678 for Python parser
Merge branch 'issue-4678-thousands-date-conflict' of https://github.com/guyrt/pandas into guyrt-issue-4678-thousands-date-conflict

Conflicts:
	doc/source/release.rst