add host based connection pool map

[hanzhumsft]

This change is for the http_client implementation on Linux.

• Add a map for host to connection pool mapping.
• Asio connection pool is not destroyed when http_client object is destroyed.
• Multiple http_client objects can use the same connection pool if the base_uri is the same.

The change can make the following code running efficiently on Linux.

make_request(/*a list of requests*/ l)
{
    request r = l.head;
    http_client client(r.request_uri().authority());
    client.request(r).then( ...; make_request(l.tail); );
}

Originally, the following code will create a connection, close it, create another connection, and so on ...
Now the connections can be reused if the authority is same.

On Windows, connection is always reused (based on host) and managed by WinHTTP. This change makes it easier for cross-platform projects to use the same piece of code.

[msftclas]

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[ras0219-msft]

Why is this change needed?

[hanzhumsft]

This is to prevent a dangling reference, because the lifetime of asio_connection_pool can be longer than the function object m_ssl_context_callback references. That function object comes from http_client_config when creating a http_client object.

[hanzhumsft]

Running tests on Linux:

**** outside_tests:outside_cnn_dot_com FAILED ****

**** outside_tests:outside_google_dot_com FAILED ****

**** outside_tests:multiple_https_requests FAILED ****

**** outside_tests:reading_google_stream FAILED ****

Finished running all 777 tests.

All 4 cases failed without my change (1e0b5c5).

[ras0219-msft]

[minor] This branch of the if already has guaranteed there is no ssl_context_callback, so passing in nullptr here is probably more clear.

[hanzhumsft]

It is possible that the ssl_context_callback does not have a valid target.

[ras0219-msft]

This will allocate a new pool shared_ptr if there is not a map entry. It would probably be better to use .find(pool_key) to avoid allocating a new map entry just to erase it.

[hanzhumsft]

Good point.

[hanzhumsft]

Instead of having a "pool of pools" and a timer for each asio_connection_pool, I made the existing asio_connection_pool have dual functionalities. With a std::multimap it can serve as a global pool shared by all asio_client. With a std::set, it also can be a private pool for just one asio_client if the client has ssl_context_callback defined.