[libcurl connection pool] Add clean up routine and timmer

[vhvb1989]

Add a background routine to clean up the connection pool.
This requires to add a time_point to each connection.

Routine should run one time after some amount of time (say 2 minutes).
Then, routine must loop all connections lists from back to front and remove all expired connections up the the first not expired connection.
When it finishes, it should set the next clean up routine to start in 2 minutes from there.

Time interval can be set up as constant in curl.hpp details