Clean routine

[vhvb1989]

fixes: #504

This PR:

• Separates CurlSession from CurlConnectionPool and CurlConnection.
• Introduces a clean thread function which gets started as soon as a connection is moved to the connection pool.
  • it runs every 5 minutes and remove any connection expired (on connection pool for more than 1 min)
  • if the clean thread removes all connections, it stop the cleaning until the next time a connection is moved to the connection pool again. This way it is not just running forever when it is not needed.

[Jinming-Hu]

@vhvb1989 can you merge this PR ASAP? we're going to release on Aug 27 (Shanghai local time).

[Jinming-Hu]

This is an infinite loop. How does it stop when the program exits?

[Jinming-Hu]

This might crash the whole program

[vhvb1989]

it gets stop as soon as there are no connections. If the clean routine removes all connections, the function returns.

if (CurlConnectionPool::s_connectionCounter == 0)
        {
          // stop the cleaner since there are no connections
          CurlConnectionPool::s_isCleanConnectionsRunning = false;
          return;
        }

this way, is the cleaner removes all connections, it would wait and if in the next check there are no connections it returns and thread finishes

[Jinming-Hu]

but still, you didn't join the thread