confapplier: introduce automatic schema upgrade

This patch introduces automatic schema upgrade. The main idea to
apply box.schema.upgrade on the leader instance.

We can't do it on each instance - it lead to replication
conflicts. We can't delegate it to users becouse a bug: "Space 277
doesn't exist" (tarantool/tarantool#4691)
still is not resolved.

Closes #640

cartridge/confapplier.lua

@@ -246,6 +246,25 @@ local function apply_config(clusterwide_config)
     return true
 end
 
+local function cartridge_schema_upgrade(clusterwide_config)
+    -- This was done in such way for several reasons:
+    --  * We don't have a way to check is current schema version is latest
+    --    (https://github.com/tarantool/tarantool/issues/4574)
+    --  * We run upgrade only on the "leader" instance to prevent replication conflicts
+    --  * We run upgrade as soon as possible to avoid Tarantool upgrade bugs:
+    --    (https://github.com/tarantool/tarantool/issues/4691)
+    --  * Also we do it completely automatically to protect user from the bug
+    --    mentioned in previous point
+    local topology_cfg = clusterwide_config:get_readonly('topology') or {}
+    local instance_uuid = box.info.uuid
+    local server = topology_cfg.servers[instance_uuid]
+    local leader_uuid = topology.get_leaders_order(topology_cfg, server.replicaset_uuid)[1]
+    local is_leader = leader_uuid == instance_uuid
+    if is_leader then
+        box.schema.upgrade()
+    end
+end
+
 local function boot_instance(clusterwide_config)
     checks('ClusterwideConfig')
     assert(clusterwide_config.locked)
@@ -406,6 +425,8 @@ local function boot_instance(clusterwide_config)
         local read_only = box.cfg.read_only
         box.cfg({read_only = false})
 
+        cartridge_schema_upgrade(clusterwide_config)
+
         BoxError:pcall(
             box.schema.user.passwd,
             username, password