Storage access caching (sdk-a)

[stneng]

Migrated from: https://github.com/CoLearn-Dev/colink-sdk-a-rust-dev/issues/2

Idea

We can reduce the protocol execution communication cost by caching some of the storage content used during the computation. Because we have read-only variables with globally unique naming, we can cache the key-value pairs.
Specifically,

• The server can cache external (in variable transfer protocol) variables transferred from other parties.
• The SDK can cache variables used during execution in memory to avoid communicating with the server.