services/horizon: Protect 'currentState' variable using Mutex to prevent race condition.

[urvisavla]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Protect the currentState variable using a Mutex to avoid a race condition caused by reading the currentState variable from UpdateCoreLedgerState() in one goroutine while updating it in the ingestion loop from another goroutine.
Since the ingestion loop runs only every few seconds and UpdateCoreLedgerState() executes once per second, the performance impact is expected to be minimal.

Why

see #4888

[urvisavla]

@tsachiherman I'm not sure how to test this race condition in unit test but the integrations test with -race seem to be passing.

[tsachiherman]

@tsachiherman I'm not sure how to test this race condition in unit test but the integrations test with -race seem to be passing.

I was hoping for something in the lines of

func TestCurrentStateRaceCondition(t *testing.T) {
	config := Config{
		CoreSession:              &db.Session{DB: &sqlx.DB{}},
		HistorySession:           &db.Session{DB: &sqlx.DB{}},
		DisableStateVerification: true,
		HistoryArchiveURLs:       []string{"https://history.stellar.org/prd/core-live/core_live_001"},
		CheckpointFrequency:      64,
	}

	sys, _ := NewSystem(config)
	s := sys.(*system)

	timer := time.NewTimer(2000 * time.Millisecond)
	getCh := make(chan bool, 1)
	doneCh := make(chan bool, 1)
	go func() {
		var cur stopState
		for range getCh {
			s.runStateMachine(cur)
		}
		close(doneCh)
	}()
forloop:
	for {
		s.GetCurrentState()
		select {
		case <-timer.C:
			break forloop
		default:
		}
		getCh <- true
	}
	close(getCh)
	<-doneCh
}

i.e. emulate what happened in this use case and show that the unit test ( which used to fail ) isn't failing anymore.