Investigate usage of str_free vs. free

Originally highlighted in https://github.com/mongodb/mongo-php-driver/pull/1075#discussion_r367770614:

While looking into this, I think I found a few places where we might just be using efree() on something returned from php_array_fetch_string. php_phongo_fetch_ssl_opt_string may be one instance where we tend to use str_efree but may use efree directly at least once.