[swh-graph] Configure the max_memory to use

The default configuration is used on granet when swh-graph start.

A configuration file should be specified in the service to allow some tuning of the service, like the max_memory used by the grpc backend. By default, 90% of the memory is allocated to the java process which is too much regarding the off-heap allocations and the other services running on the server.

swhwork+   10051   10043 99 07:42 ?        06:13:35 java -cp /opt/swhgraph_venv/share/swh-graph/swh-graph-1.0.1.jar -Xmx729176787763 -XX:PretenureSizeThreshold=512M -XX:MaxNewSize=4G -XX:+UseLargePages -XX:+UseTransparentHugePages -XX:+UseNUMA -XX:+UseTLAB -XX:+ResizeTLAB org.softwareheritage.graph.rpc.GraphServer --port 50091 /dev/shm/swh-graph/default/graph

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Migrated from T4509 (view on Phabricator)