Add a timeout method applying to the whole session

Provides a solution for the issue #83115 on the rt.cpan.org tracker (https://rt.cpan.org/Public/Bug/Display.html?id=83115).

> Dear all,
> 
> Having a way to set a global timeout allows to avoid long freezes when
> network errors occur.
> 
> If I unplug the network cable after being connected and then try to open()
> a file through SFTP, Net::SSH2 takes about 15 minutes to detect there is a
> problem and returns.
> 
> libssh2 (version 1.2.9) provides libssh2_session_set_timeout() which allows
> to set a timeout that will affect every action on the session.
> 
> Best,
> 
> Thierry Treyer