add a TTL to transaction cache entries

[edaniszewski]

Description

Reading through the code, it looks like there is no TTL set when transactions are added to the transaction_cache. We'll probably want that to prevent it from getting massive..

My initial thought is that the TTL can be configurable, and if it doesn't exist in the config, it would fall back to some default. Just need to figure out what a sane default value is.