Rename "replica weights" to "distances"

[rosik]

Vshard has two "weight" properties - replica weights and replica set weights. But their meaning is completely different.

• https://www.tarantool.io/en/doc/latest/reference/reference_rock/vshard/vshard_admin/#replica-weights
• https://www.tarantool.io/en/doc/latest/reference/reference_rock/vshard/vshard_admin/#replica-set-weights

Instead of "replica weight" we should call it "distances", because it better reflects its purpose.

Let's rename it in code and in doc. But do it in compatible manner.

[rosik]

I'm opening this issue for two reasons:

1. As we recently discussed in context of trainings, two weights with a different meaning are obscure.
2. We're going to support it in cartridge (https://github.com/tarantool/cartridge/issues/201), and if there is a chance it'll ever be renamed, it's better to come to agreement right now.