I keep my MBA 2013 plugged in most of the time. AFAIK for a long term it is better to keep modern battery around 40%. I have seen that some other laptop manufacturers added feature to their drivers to prevent battery charging over certain level. Is there anything similar available for mac?
The 40% charged state is applicable for Lithium-Ion batteries only if you're going to remove the battery and put it in long term storage without using it. Refer How to Prolong Lithium-based Batteries. In the case of MacBook models, Apple recommends storing the batteries at 50% charge if you do not plan to use it for more than six months.
For normal usage, Apple recommends that you run the laptop on battery once in a while and also do a full discharge and recharge to calibrate the battery (full discharge/recharge means running it down till the computer automatically goes to sleep or shuts down due to low battery and then recharging it completely to 100% in one go).
For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her notebook on the train, then plugs it in at the office to charge. This keeps the battery juices flowing. If on the other hand, you use a desktop computer at work, and save a notebook for infrequent travel, Apple recommends charging and discharging its battery at least once per month.
See Apple's guide on Notebook batteries for detailed information.
Not to my knowledge. The MacBooks already do some management of charging, for example if the battery is almost full it will not start charging to 100%, to save charging cycles.
Also, to my knowledge modern batteries die by age, mostly independently to the charging and discharging cycles. But I might be wrong there.