Use simple growth scale with any max size for BufferedRowResultProxy

Use simple growth scale with any max size for BufferedRowResultProxy

The maximum buffer size for the :class:`.BufferedRowResultProxy`, which
is used by dialects such as PostgreSQL when ``stream_results=True``, can
now be set to a number greater than 1000 and the buffer will grow to
that size.  Previously, the buffer would not go beyond 1000 even if the
value were set larger.   The growth of the buffer is also now based
on a simple multiplying factor currently set to 5.  Pull request courtesy
Soumaya Mauthoor.

Fixes: #4914
Closes: #4930
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4930
Pull-request-sha: 66841f56e967c784f7078a787cec5129462006c8

Change-Id: I6286220bd9d488027fadc444039421a410e19a19