Ability to insert duplicate keys in unique index

[0x501D]

If not using the most efficient representation we can insert duplicate keys in unique index:

Tarantool 2.11.2-0-g1bac2d2

tarantool> s = box.schema.space.create('test')
---
...

tarantool> s:create_index('pk')
---
- unique: true
  parts:
  - type: unsigned
    is_nullable: false
    fieldno: 1
  hint: true
  id: 0
  type: TREE
  space_id: 512
  name: pk
...

tarantool> msgpack = require('msgpack')
---
...

tarantool> s:insert{1}
---
- [1]
...

tarantool> o = msgpack.object_from_raw('\x91\xcf\x00\x00\x00\x00\x00\x00\x00\x01')
---
...

tarantool> o:decode()
---
- [1]
...

tarantool> s:insert(o)
---
- [1]
...

tarantool> s:select()
---
- - [1]
  - [1]
...

[unera]

I think that it can be frequent pattern. For exampe a user could use Java messagepack.encoder and tarantool's.

So if a user write into one space using two languages, he can stumble over the bug.

[R-omk]

Maybe related
tarantool/vshard#207