services/horizon: Split App.UpdateLedgerState into Core and Horizon ledger update methods

[bartekn]

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

This commit splits UpdateLedgerState into UpdateCoreLedgerState and UpdateHorizonLedgerState which update Core and Horizon ledger state separately. It also changes Tick frequency to 5 seconds.

Close #3883.

Why

If Stellar-Core is unresponsive the call to Stellar-Core /info endpoint will be cancelled after 10 seconds but it will also return entire UpdateLedgerState method. This will make Horizon ledgers stats out of date which can affect streaming (because streams are updated only when Horizon ledger in ledger.Status is incremented.

Known limitations

With this change it's possible that Horizon root can report Core ledger sequence behind Horizon.