Add support for 'docker exec' - phase 2

Depends on #7409. 

Daemon now supports resizing of tty sessions used for exec. 
'exec' Remote API introduced in #7409 has been split into '/containers/<name>/exec' and '/exec/<name>/start'.
'/exec/<name>/resize' has been added to support resizing of exec tty sessions.

I tried to avoid unnecessary code duplication.