Get core latest ledger from http end-point instead of core db.

[abuiles]

We are in the process of removing Horizon's dependency on Stellar Core database. This is one of the places where we still use it.

go/services/horizon/internal/app.go

Lines 204 to 236 in 28355c0

	func (a *App) UpdateLedgerState() {
	var next ledger.State
	logErr := func(err error, msg string) {
	log.WithStack(err).WithField("err", err.Error()).Error(msg)
	}
	err := a.CoreQ().LatestLedger(&next.CoreLatest)
	if err != nil {
	logErr(err, "failed to load the latest known ledger state from core DB")
	return
	}
	err = a.HistoryQ().LatestLedger(&next.HistoryLatest)
	if err != nil {
	logErr(err, "failed to load the latest known ledger state from history DB")
	return
	}
	err = a.HistoryQ().ElderLedger(&next.HistoryElder)
	if err != nil {
	logErr(err, "failed to load the oldest known ledger state from history DB")
	return
	}
	next.ExpHistoryLatest, err = a.HistoryQ().GetLastLedgerExpIngestNonBlocking()
	if err != nil {
	logErr(err, "failed to load the oldest known exp ledger state from history DB")
	return
	}
	ledger.SetState(next)
	}

In the code above, instead of reading from core's database, we should fetch this value from Horizon's DB - we discuss the possibility of adding this value to the key_value_store and have the front-end instances fetch the value from there.

We'll put the responsibility of updating this value in the ingesting instances. We can take the following approach:

1. Use core's http end-point if Horizon if Horizon is still syncing/ingesting.
2. Once Horizon has catched up with core, update the value every time a new ledger is processed instead of reading from core's http end-point.

What alternatives are there?

Remove this field from Horizon's root resource.