[FIX] export performance regression from d28f8f7
[FIX] export performance regression from d28f8f7

d28f8f704febe030f9c068f895adf0b1ce3e8dc1 carefully clears only the
relevant bit of the cache (only the top-level object which we're
exporting), however the commit did not considert that _export_rows
calls itself recursively for relational fields, and thus when
exporting relational fields the relation would become the new
top-level and get cache-cleared on every iteration, significantly
slowing down cases where such records are shared and would need to be
re-fetched every iteration, even more so if they're somewhat/somehow
expensive.

OPW-1833545
OPW-1836361

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo