Being able to set an expiration time per key, and not just per dict

Currently it's only possible to set an expiration time to the entire dictionary when creating a new instance:

local obj = resty_lock:new("locks_dict", {exptime = 10})

It would be nice to allow setting an expiration time per key too, like:

local obj = resty_lock:new("reports_locks")
obj:lock("test", {exptime = 10})

So that one dictionary can be used to set multiple keys with a different expiration time:

local obj = resty_lock:new("reports_locks")
obj:lock("test", {exptime = 10})
obj:lock("test2", {exptime = 4})
obj:lock("test3", {exptime = 50})

Start at the resty_lock:new and obj:lock entry points, then trace how the dictionary-level exptime is applied to shared-memory dictionary entries. Done means the lock API accepts the per-key options shown and allows different expiration values for keys in one dictionary; verify the behavior with the relevant lock tests if present.