Free reservation is called each time an extra quantity is
used on a move (and for each immediate transfer). It means
that the function is called many times on a database with an
inventoy activity. Also _free_reservation method do a search
on fields that are not indexed, and is slow on large database.

This index is specific to _free_reservation.
On a live database with an important stock, the queries dropped from 265ms
to 1.5ms.

closes odoo/odoo#64311

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>