store-db: set autovacuum_work_mem to 10% of memory

We have seen host crashes caused by PostgreSQL using up all memory by
trying to run `autovacuum` workers.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ansible/group_vars/store-db.yml

@@ -28,6 +28,7 @@ postgres_alter_system_settings:
   max_wal_size: '1GB'
   min_wal_size: '80MB'
   max_locks_per_transaction: '2160'
+  autovacuum_work_mem: '{{ ((ansible_memtotal_mb * 0.1) * 1000) | int }}' # kB
 
 # Open PostgreSQL Port
 open_ports_default_comment: '{{ postgres_ha_service_name }}'