Cache encryption proofs for verification

[AddressXception]

Bug Report

Current Behavior
When provisioning trustees, encrypting ballots, and decrypting tallies, proofs are generated and checked in memory but they are not persisted for use when publishing for verification

Expected behavior/code
Proofs should be persisted for use with election verifiers

Environment
all

Possible Solution
cache the proofs using the same mechanism as used for each data type (e.g. the file system)