vshard: introduce connection_fetch_schema config option

[TarantoolBot]

Root document: https://www.tarantool.io/ru/doc/latest/reference/reference_rock/vshard/vshard_ref/

A new configuration option connection_fetch_schema has been added.

It controls whether vshard connections should fetch the database schema
(to allow using conn.schema.<...> or conn.space.<space_name>).

vshard itself doesn’t need this fetching at all, so you can safely set
connection_fetch_schema = false - as long as you’re not relying on its
internal connections. This will improve performance.

By default, it’s set to true (enabled) for backward compatibility.
Requested by @Astronomax in tarantool/vshard@c83deb1.

[veod32]

@lenkis Please estimate