Refactor ScriptInvoker to handle a TODO related to Direct and to anticipate multiple entrypoints on the same script library.
Introduces a new pooled object, ScriptLibrary, that represents the invocable script rather than any particular entrypoint.
merge mmcm: Clean up ScriptInvoker as mentioned in lp:~pbeaman/akiban-server/direct-param-handling/+merge/156711/comments/343411

https://code.launchpad.net/~mmcm/akiban-server/script-invoker-pool/+merge/158507