It should consider characters in unicase scripts as being not uppercase.

Issue: #4947
Closes: #4947

Issue: #4929

This reverts commit 5e84a82f87.

In fact, we cannot send multiple commands and read them through the pipe. It has
caused multiple weird issues in the past but I can now see clearly how it should
be done. We should send commands and serialize all requests pending to let them
being sent one by one, after reply for the previous command has been received.

Issue: #4919

Treat relearning from ham to spam the same as relearning from spam to ham. `res` is only set to true if the mail is already learned.

If we detected extension of the file and this extension is equal to the real extension then we should not pass second extension to check function as it will trigger false positive for MIME_DOUBLE_BAD_EXTENSION.

Issue: #4908
Closes: #4908

Issue: #4892
Closes: #4892

Issue: #4885
Closes: #4885

We should do like MUAs do: imlicitly close incomplete mime parts
and treat them as ususal. It is weird and utterly stupid but that's
what MUA do.

Issue: #4740
Closes: #4740

This types have constant conflicts with the system ones especially on OSX.

No reasons why not to do it, as there are no supported versions without compression support

Rspamd currently sends reply in JSON format. However, message pack seems to
be a better choice for a compatible client. It is faster and does not need
escaping or even UTF8 validation. This is a surface for further updates.

We check userdata very frequently, so the idea here is the following:

- Store the address of classname (converted to int) in the metatable at index 1
- When we need to check some udata, we can just compare the static address with
  the integer stored in metatable
- This avoid quite an expensive `lua_rawequal` call for two tables as we know that
  our classes are quite static