By Jamie Spencer Liverpool’s long-serving goalkeeping coach John Achterberg has announced his decision to leave the club at the end of the season in a further shakeup to the backroom staff. Achterberg has been a coach at Liverpool since 2009, starting with the academy before joining the first-team staff in the summer of 2011. During that time, the 52-year-old has served under three different managers in Sir Kenny Dalglish, Brendan Rodgers and Jurgen Klopp. But rather than continue into a fourth reign when compatriot Arne Slot arrives from Feyenoord this summer, Achterberg has opted to follow K…