Limit koschei-resolver memory usage

This limits real memory usage, virtual memory is not limited -
resolver won't use more than 2G of *real* memory (RAM), virtual memory
exceeding that limit will be swapped to disk.

This should leave more real memory for other services and disk cache,
while allowing koschei-resolver to keep older sacks in swap (loading
sack from swap should be faster than reloading it from repo files).

roles/koschei/backend/files/systemd-memory-limit.conf

@@ -0,0 +1,2 @@
+[Service]
+MemoryLimit=2G

roles/koschei/backend/tasks/main.yml

@@ -21,6 +21,17 @@
   - koschei
   - packages
 
+- name: install systemd config files
+  copy: >
+    src=systemd-memory-limit.conf
+    dest=/etc/systemd/system/koschei-resolver.service.d/memory-limit.conf
+  notify:
+  - reload systemd
+  - restart koschei-resolver
+  tags:
+  - koschei
+  - config
+
 - name: install config files
   when: env == "staging"
   template: >