Add ledger close dates to transaction, operation, and effect resources

[nullstyle]

This will allow someone to, for example, build a transaction history with age display without having to load date data afterwards

[bartekn]

Can we try to close this issue? I'm building an app that has a view with 200 recent operations. Loading tx time for each operation is an issue because it generates another 200 requests to horizon what makes the app very slow. It wouldn't be that big problem if it was a single page but my view also allows to go to the next page of operations. Then, if user navigates to the next page while the current page is still loading, live requests to /transactions should be cancelled and it's a pain.

@nullstyle I can do it if you don't have time (in a similar way to #345).

[nullstyle]

That seems smart to me. I can build this today @bartekn. IMO we need to separate the ledger loader/cache out of the trade endpoint and reuse it across the others.

[nullstyle]

I'm just finishing up this issue today... the ledger caching system I'm building needs some tests and then I can roll it out to other actions quite easily.