Ensure that transient redis errors do not kill workers.
Remove the lua script cache.

(I'm going to implement a better solution next).
Only load lua scripts if they are needed.

Given how many workers we are and how rarely the scripts change,
most of the time the lua scripts do not need to be loaded when
a client is created.  It can be delayed until it is actually
needed (often never during the lifetime of the process).
Merge pull request #90 from seomoz/harden_worker

Harden worker and improve script caching