Add setMaxLockDuration function to add-veLON-impplementation

[zeqing-guo]

Add setMaxLockDuration function to add-veLON-impplementation branch.

Given the correctness and fairness, the code guarantees that if old_power_A > old_power_B, then after updating the maxLockDuration we have new_power_A >= new_power_B.

Code details are as follows:

• if NFT's lock.end > maxEndTime, we set the lock.end = maxEndTime. Then we update NFT's slope and bias
• if NFT's lock.end <= maxEndTime, we update NFT's slope and bias directly

After updating every NFT's end, slope, and bias, we will update the global slope and bias.

[charlesjhongc]

No particular issue at the first glance. However this function may consume lots of gas and perhaps exceed block limit? Need some test for gas consumption.

[charlesjhongc]

This one can be closed since we decided to use fixed maxDuration instead.