memory leak in CLI CSV import

There's a significant memory leak in the API that primarily manifests during CSV CLI import (bin/csv/import.php) - e.g. it leaks about 60MB for every 1000 contact records imported.  This makes large imports impractical.

The leak is cleared by unsetting the globals used by the PEAR_DB_DATAOBJECT cache.

To reproduce:

Add this line to <civiroot>/bin/class.cli.php after line 432 ($this->row++)

echo 'Memory usage: ' . memory_get_usage(TRUE) / 1048576 . PHP_EOL;

Then attempt to import a large number of contacts. 

php csv/import.php -e Contact --file /path/to/contacts.csv

The solution is to unset the globals every so many API calls. PR incoming.