Gradually increasing server lag ends in crash

Server lag gradually increases over time, eventually leading to a crash.

I usually just restart the server when lag becomes unbearable. I can play for several hours without noticeable lag when I'm alone on server. Issue escalates very quickly as the number of player increases.

Strangely enough, placing water sources seems to accelerate the problem, as I get intense lag spikes (server-side lag, not client-side) when placing source blocks, no matter where.

Reverting server to 15w47c usually solves the problem.

Attached crash log.