Clean up multi_handles on the same thread

[clintonpi]

Motivation:

URLSession elusively crashes due to a race condition in OpenSSL <1.1.0 when calling curl_multi_cleanup concurrently in different threads.

Partially resolves #3813

Modifications:

• Adjust _MultiHandle.deinit to asynchronously queue curl_multi_cleanup calls in _MultiHandle.queue.

Result:

Saves OpenSSL from cleaning up concurrently in different threads, avoiding the crash.

Testing:

Passing existing tests is sufficient. However, without this patch, the code snippet below crashes about once in 20 runs on Amazon Linux 2

curl 8.3.0 (aarch64-koji-linux-gnu) libcurl/8.3.0 OpenSSL/1.0.2k-fips zlib/1.2.7 libidn2/2.3.0 libpsl/0.21.5 (+libidn2/2.3.0) libssh2/1.4.3 nghttp2/1.41.0 OpenLDAP/2.4.44
Release-Date: 2023-09-13

DispatchQueue.concurrentPerform(iterations: 100) { _ in
    let session: URLSession = URLSession(configuration: URLSessionConfiguration.default)
    DispatchQueue.concurrentPerform(iterations: 100) { _ in
        _ = session
    }
}