Pixel tracking retention should be time based, not record count based

[dev-dull]

The current implementation of the pixel tracking is based on how many records should be kept. Instead, it should be based on the age of the data.

To put it a second way, default to keeping data for 30 days instead of keeping 10000 records.

[dev-dull]

Here is where the check is performed:

PyXIE/ddb.py

Line 239 in 0d97899

while len(self) > self._max_size:

While you're at it, you can likely remove the other _cleanup() method in the same file.