net.box: remove varargs from call() and eval()

Change conn:call() and conn:eval() API to accept Lua table instead of
varargs for function/expression arguments:

    conn:call(func_name, arg1, arg2, ...)
      =>
    conn:call(func_name, {arg1, arg2, ...}, opts)

    conn:eval(expr, arg1, arg2, ...)
      =>
    conn:eval(expr, {arg1, arg2, ...}, opts)

This breaking change is needed to extend call() and eval() API with
per-requests options, like `timeout` and `buffer` (see #2195):

    c:call("echo", {1, 2, 3}, {timeout = 0.2})

    c:call("echo", {1, 2, 3}, {buffer = ibuf})
    ibuf.rpos, result = msgpack.ibuf_decode(ibuf.rpos)
    result

Tarantool 1.6.x behaviour can be turned on by `call_16` per-connection option:

    c = net.connect(box.cfg.listen, {call_16 = true})
    c:call('echo', 1, 2, 3)

This is a breaking change for 1.7.x.

Needed for #2285
Closes #2195