ingest/ledgerbackend: Fix captive core bug where --start-at-hash parameter is omitted

[tamirms]

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

In #3201 we assumed it was possible to omit the --start-at-hash parameter when starting from the genesis ledger. However, Stellar Core actually fails when attempting that command. The --start-at-hash parameter is always required when using --start-at-ledger.

This means that if we want to stream from the genesis ledger we need to determine the either the horizon db or the history archives.

In the case that the first checkpoint has not been published we attempt to invoke stellar-core run --in-memory without any start parameters. The expected behavior from stellar core is that it will join the network and stream from the nearest checkpoint boundary, which in this case would be the genesis ledger since the first checkpoint has not been published yet.

[edit:] I tested the behavior of stellar-core run --in-memory on a stand alone network and unfortunately stellar core will keep buffering ledgers until the first checkpoint is published before it starts streaming any ledgers. Therefore it does not make any sense to use this work around.

Known limitations

N / A