feat(lock) exptime can be set per key

[subnetmarco]

Closes #7.

[subnetmarco]

@agentzh any comments on this PR?

[agentzh]

I think it's a bit confusing to use the two words "timeout" and "exptime" in the same API since they are essentially the same thing in this context. I think you are actually proposing a per-key timeout value here, right? Then we can just accept a "timeout" argument in lock() to simply override the object-level default timeout setting. What do you think?