Add use_nchar_for_unicode flag; don't use nchar types for generic unicode

Add use_nchar_for_unicode flag; don't use nchar types for generic unicode

The Oracle dialect will no longer use the NCHAR/NCLOB datatypes to
represent generic unicode strings or clob fields in conjunction with
:class:`.Unicode` and :class:`.UnicodeText` unless the flag
``use_nchar_for_unicode=True`` is passed to :func:`.create_engine`.
Additionally, string types under Oracle now coerce to unicode under Python
2 in all cases, however unlike in previous iterations, we use SQLAlchemy's
native unicode handlers which are very performant (when C extensions are
enabled; when they are not, we use cx_Oracle's handlers).

Change-Id: I3939012e9396520875bc52e69bf81f27393836ee
Fixes: #4242