Reuse module cache across transactions and ledgers

[graydon]

This is a broad issue that touches on both stellar-core and soroban: to attempt to make the module cache in soroban -- that stores contracts intheir translated / ready-to-run form -- retain its content and be reused across transactions, and even across ledgers. Doing so will significantly lower costs and increase throughput.

In the past this has been blocked on figuring out a suitable cost and fee model for cache misses and (re-)population events. Recently we've been considering a simplified approach where all live (non-archived) modules are assumed to be in cache at all times, so fees only need to be charged at upload or archival-restoration.

We have preliminary prototypes of this working, but there are a lot of details to work out still.