Ref: https://mojang.atlassian.net/browse/MC-749

Attempt to fix the bug where, if the last trade of a villager gets locked, he can't get new trades.

Every trade, old or new, should have a chance to renew other trade or create a new one.

At a random amount of time, some Villagers with locked trades could get 1~3 more uses of that trade (higher chance if village reputation is good?).