Currently there is the following issue with cached conversion rates
from the "from currency" to the "to currency".
When the rate of the res_currency_rate of the "to currency" changes the cache is not invalidated.
Thus the cached value may be used wrongly (leading to wrong results).
This commit simply invalidates all the cached rates in case we update or
create a res_currency_rate.
closesodoo/odoo#143847
Signed-off-by: Rémy Voet (ryv) <ryv@odoo.com>