Avoid race conditions

[argaen]

There are a couple of places at least identified where race conditions can occur:

• cached/multi_cached decorators between get and set Race condition in @cached #95
• increment CMD for memcached between incr and set.

Once #27 or #115 are implemented, should use the features to let the user choose whether to live or not with the possible race conditions.

TODO:

• cached support for redlock
• multi_cached support for redlock
• increment: won't do

[argaen]

With #27 closed:

• cached decorator can support now redlock algorithm to avoid stampede effects
• I won't add support for multicached yet, need to tweak implementation a bit...
• for increment command I'll leave it as it is for now.

[argaen]

I'll wait for someone to request for this functionality, don't want to implement this feature for now