Add bucket_id calculation function

[Totktonada]

After #71 we'll have all information that is necessary for calculating bucket_id by a sharding key. The proposal is to implement this helper function in the module.

Example

Schema:

TBD

Assume we serve HTTP requests:

function handler(req)
    local bucket_id = ddl.bucket_id('users', req:json()['user_id'])
    local ok, err = vshard.router.callrw(bucket_id, <..args..>)
    <...>
end

The key point is that ddl knows, which sharding function is is use. Without the function we would do the following:

function handler(req)
    local bucket_id = vshard.router.bucket_id_mpcrc32(req:json()['user_id']) -- !!
    local ok, err = vshard.router.callrw(bucket_id, <..args..>)
    <...>
end

[ligurio]

Discussed verbally with @Totktonada:

• name of the new method is bucket_id (not get_bucket_id)
• user pass sharding key that is ready for use in bucket_id
• it should not be a default for hash function (in a future it would be nice to have it, but now with introducing the default hash function we may break DDL users)

[ligurio]

Blocked by #71