New InputBuffer class should release data for DBF when DBF is closed

[RobertvanderHulst]

The new inputbuffer class for DBF files keeps the data for tables that are closed.
A customer has code that overwrites DBF file after closing it and after reopening the file the RDD uses the "old" data and not the new data from the file that did overwrite the original file.

[RobertvanderHulst]

Fixes in e16e4a9

[cpyrgas]

Robert, looks good, but can't reproduce this problem with the previous version of the runtime either, so I will let you close it.

[RobertvanderHulst]

I have created a unit test for this. The problem happens when the file is closed, overwritten and reopened when the GC had no time to clear the buffer from the cash. In that case the new file was "inheriting" the old buffer