Caching of treasury state

[dkaidalov]

In some cases, when the info from past treasury epochs is needed, it is retrieved directly from the blockchain that is stored on disk. It is very inefficient and can fail under huge load.
In many cases, this information can be extracted only one time and cached for future use, probably even before it is really needed.
This mostly affects generation of randomness which requires actions of the committee members from the previous epochs.