TcpSocket#Bind(): also set SO_REUSEPORT

[Al2Klimov]

fixes #6898

[dnsmichi]

Relatively new with Kernel 3.9 (that's why I didn't know it existed). Reading about it makes me believe that this could solve problems with the API thread pool later on as well.

https://stackoverflow.com/questions/14388706/socket-options-so-reuseaddr-and-so-reuseport-how-do-they-differ-do-they-mean-t
https://medium.com/uckey/the-behaviour-of-so-reuseport-addr-1-2-f8a440a35af6

http://man7.org/linux/man-pages/man7/socket.7.html

        SO_REUSEPORT (since Linux 3.9)
              Permits multiple AF_INET or AF_INET6 sockets to be bound to an
              identical socket address.  This option must be set on each
              socket (including the first socket) prior to calling bind(2)
              on the socket.  To prevent port hijacking, all of the pro‐
              cesses binding to the same address must have the same effec‐
              tive UID.  This option can be employed with both TCP and UDP
              sockets.

              For TCP sockets, this option allows accept(2) load distribu‐
              tion in a multi-threaded server to be improved by using a dis‐
              tinct listener socket for each thread.  This provides improved
              load distribution as compared to traditional techniques such
              using a single accept(2)ing thread that distributes connec‐
              tions, or having multiple threads that compete to accept(2)
              from the same socket.

              For UDP sockets, the use of this option can provide better
              distribution of incoming datagrams to multiple processes (or
              threads) as compared to the traditional technique of having
              multiple processes compete to receive datagrams on the same
              socket.

[dnsmichi]

Thanks 👍