vshard.storage.bucket_stat() fails when called in trigger on vinyl space.

[aensidhe]

Setup

• I have vshard, properly bootstrapped.
• I have sharded space with vinyl engine.
• I have on_replace trigger on that space.
• I call vshard.storage.bucket_stat() to understand whether it is data change caused by the application or it is vshard rebalancing

Expected

I get the info about bucket

Actual

vshard.storage.bucket_stat() crashes with error: A multi-statement transaction can not use multiple storage engines

Important note: if my space would have memtx engine, everything works perfectly.

How to reproduce

1. Bootstrap vshard any way you want.
2. Connect to master of any replicaset.
3. Setup test space:

   box.schema.create_space('test', { engine = 'vinyl' })
   box.space.test:format{ { 'name': 'id', 'type': 'unsigned' }, { 'name': 'bucket_id', 'type': 'unsigned' } }
   box.space.test:create_index('primary', { parts = { 'id' } })
   box.space.test:create_index('bucket_id', { parts = { 'bucket_id' }, unique = false })
   function f(old, new) print(vshard.storage.bucket_stat((old or new).bucket_id)) end
   box.space.test:on_replace(f)

4. Run the test: box.space.test:insert{1, 1}
5. See the error: error: A multi-statement transaction can not use multiple storage engines