Implement in-memory wal-free bucket references

[Gerold103]

It is two counters: RO and RW. RO reference counter protects garbage bucket from being deleted while it has some active requests working with it. RW reference counter will protect bucket from relocation by a rebalancer or an explicit bucket_send, while it has active read-write requests. References are not persisted since they count number of active requests, but on restart all requests are dropped. Bucket refs should have a public API for manual work, and maybe an automatic ref/unref via IPROTO option, or via a special wrapper function, like vshard.storage_call.
Please, note that request != transaction. A request is usually a stored procedure call, making a lot of transactions on a bucket. When a request works with multiple buckets, it should be able to reference each of them via public API before usage. In vshard it looks like this: vshard.storage.bucket_refro(10) --do some work, many transactions-- vshard.storage.bucket_unrefro(10). A user expects that a bucket will not be relocated nor deleted by GC during a request.

Follows #3986.