[baloo-file] Fix broken memory limit

The memory limit sets the combined allocation of the main baloo_file deamon, and the baloo_file_extractor including any transient allocation done by the kfilemetadata extractor plugins.

Setting a memory limit will not lower the memory taken by the extractor (which is out of control of baloo), but will cause it to hold on the memory for an extended period of time (man systemd.resource-control):

Memory usage may go above the limit if unavoidable, but the processes are heavily slowed down and memory is taken away aggressively in such cases.

This has caused several issues in the past already, where the daemon and extractor become apparently live-locked and are essentially starving to death.

There were several changes in the somewhat recent past to reduce the memory taken by baloo itself by limiting the transaction size by splitting transactions, but the current limit is way too low and may be hit by indexing e.g. a single PDF file.