Simplified file storage read() method.
Simplified static checks in Repo.
Simplified JSON->MsgPack packing.
More const-ness and proper lock types.

Apply const modifier to more types across the code, as well as change
some unique_lock<T>-s to lock_guard<T>-s where applicable.
Use const iterators where applicable.
Switch file storage find() logic to intersection.
Revert "A simple vector iterator is okay..."

This reverts commit e3c89a3ad49552dde4387a9f6f4c684a05dc7a97.
Merge branch 'master' of github.com:cocaine/cocaine-core