attr-sql: Make release of online leases during startup optional

This cleanup prevents sharing the same DB between multiple VPN gateways.

conf/plugins/attr-sql.opt

@@ -1,3 +1,7 @@
+charon.plugins.attr-sql.crash_recovery = yes
+	Release all online leases during startup.  Disable this to share the DB
+	between multiple VPN gateways.
+
 charon.plugins.attr-sql.database
 	Database URI for attr-sql plugin used by charon. If it contains a password,
 	make sure to adjust the permissions of the config file accordingly.