storage: fix services being nullified too late

If user tries to reconfigure vshard very often, it may happen, that
service didn't manage to notice, that it's cancelled, before
the new one is started. In such case the new fiber may encounter
assertion failure and will be restarted several times.

The approach, which makes the service nullification, when the current
one is the one stored globally was chosen, since with blocking service
cancellation reconfiguration of vshard may take huge amount of time.

The commit doesn't have test, because it applies the same approach,
which was introduced and tested for router in the previous commit.

Closes of #9

NO_DOC=bugfix
NO_TEST=<previous commit>

Author: Serpentian

vshard/storage/init.lua

@@ -1099,12 +1099,12 @@ local function recovery_service_f(service)
 end
 
 local function recovery_f()
-    assert(not M.recovery_service)
     local service = lservice_info.new('recovery')
     M.recovery_service = service
     local ok, err = pcall(recovery_service_f, service)
-    assert(M.recovery_service == service)
-    M.recovery_service = nil
+    if M.recovery_service == service then
+        M.recovery_service = nil
+    end
     if not ok then
         error(err)
     end
@@ -2320,12 +2320,12 @@ local function gc_bucket_service_f(service)
 end
 
 local function gc_bucket_f()
-    assert(not M.gc_service)
     local service = lservice_info.new('gc')
     M.gc_service = service
     local ok, err = pcall(gc_bucket_service_f, service)
-    assert(M.gc_service == service)
-    M.gc_service = nil
+    if M.gc_service == service then
+        M.gc_service = nil
+    end
     if not ok then
         error(err)
     end
@@ -2907,12 +2907,12 @@ local function rebalancer_service_f(service)
 end
 
 local function rebalancer_f()
-    assert(not M.rebalancer_service)
     local service = lservice_info.new('rebalancer')
     M.rebalancer_service = service
     local ok, err = pcall(rebalancer_service_f, service)
-    assert(M.rebalancer_service == service)
-    M.rebalancer_service = nil
+    if M.rebalancer_service == service then
+        M.rebalancer_service = nil
+    end
     if not ok then
         error(err)
     end
@@ -3463,12 +3463,12 @@ local function instance_watch_service_f(service)
 end
 
 local function instance_watch_f()
-    assert(not M.instance_watch_service)
     local service = lservice_info.new('instance_watch')
     M.instance_watch_service = service
     local ok, err = pcall(instance_watch_service_f, service)
-    assert(M.instance_watch_service == service)
-    M.instance_watch_service = nil
+    if M.instance_watch_service == service then
+        M.instance_watch_service = nil
+    end
     if not ok then
         error(err)
     end