Allow using is_async + on_push for net.box calls

[rosik]

Is your feature request related to a problem? Please describe.

tarantool> conn:call('sub', nil, {is_async = true, on_push = print})
---
- error: 'builtin/box/net_box.lua:1062: To handle pushes in an async request use future:pairs()'
...

Net.box connections restrict using on_push triggers for async calls. I'm convinced this restriction is unreasonable.
The on_push trigger is anyway used implicitly, and equals table.insert:

tarantool> future = c:call('sub', nil, {is_async = true})
---
...

tarantool> future.on_push == table.insert
---
- true
...

Describe the solution you'd like

Let's make this use case allowed.

[Mons]

This feature requires the deep design of async interactions with net.box and other async approaches.