httpc: fix a crash triggered by gc

Bump curl version to 8.4.0 triggers a crash in Tarantool due to commit
"h2: testcase and fix for pausing h2 streams" [1]. The original
reproducer involves etcd and an etcd-client Lua module, running
etcd-client tests as a part of Tarantool integration testing is planned
to do in scope of [1].

However, the problem could be reproduced with a Lua code below:

```
local url = 'https://google.com/'

local c = require('http.client').new()

r1 = c:get(url, {chunked = true})
r1:read(1)
r2 = c:get(url, {chunked = true})
r2:read(1)
r3 = c:get(url, {chunked = true})
r3:read(1)
r4 = c:get(url, {chunked = true})
r4:read(1)

c = nil
collectgarbage()
collectgarbage()

r1:read(1)
r2:read(1)
r3:read(1)
r4:read(1)

collectgarbage()
collectgarbage()
```

According to Curl documentation, `curl_multi_cleanup` [1] must be called
before any easy handles are cleaned up. The patch adds a cleanup of easy
handles on running `curl_env_destroy`, right before calling
`curl_multi_cleanup`. The patch uses a function 'curl_multi_get_handles'
that returns all added easy handles introduced in Curl 8.4.0. Therefore
bump to 8.4.0 is required.

1. https://github.com/curl/curl/commit/6b9a591bf7d82031f463373706d7de1cba0adee6
2. https://curl.se/libcurl/c/curl_multi_cleanup.html

Fixes #9283

1. https://github.com/tarantool/tarantool/issues/9093

NO_DOC=bugfix
NO_TEST=no simple reproducer, covered by tests in etcd-client