Net.box triggers (on_connect, on_schema_reload) are executed
by the net.box connection worker fiber so a request issued by
a trigger callback can't be processed until the trigger returns
execution to the net.box fiber. Currently, an attempt to issue
a synchronous request from a net.box trigger leads to a silent
hang of the connection, which is confusing. Let's instead raise
an error until tarantool#7291 is implemented.

We need to add the check to three places in the code:
 1. luaT_netbox_wait_result for future:wait_result()
 2. luaT_netbox_iterator_next for future:pairs()
 3. conn._request for all synchronous requests.
    (We can't add the check to luaT_netbox_transport_perform_request,
    because conn._request may also call conn.wait_state, which would
    hang if called from on_connect or on_schema_reload trigger.)

We also add an assertion to netbox_request_wait to ensure that we
never wait for a request completion in the net.box worker fiber.

Closes tarantool#5358

@TarantoolBot document
Title: Synchronous requests are not allowed in net.box triggers

An attempt to issue a synchronous request (e.g. `call`) from
a net.box trigger (`on_connect`, `on_schema_reload`) now raises
an error: "Synchronous requests are not allowed in net.box trigger"
(Before tarantool#5358 was
fixed, it silently hung.)

Invoking an asynchronous request (see `is_async` option) is allowed,
but the request will not be processed until the trigger returns and
an attempt to wait for the request completion with `future:pairs()`
or `future:wait_result()` will raise the same error.