ingest/ledgerbackend: Restart captive core when a new version of core is detected on disk

[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

Close #3602

• Add a ticker to stellarCoreRunner which will execute every 10 seconds. The stellarCoreRunner context will interrupt the goroutine attached to the ticker so that if captive core shutsdown the ticker will also stop.

• Upon every tick, we will check to see if the stellar core binary has been modified. If it has been modified we log a warning message to announce we will be shutting down captive core and then we call stellarCoreRunner.close() to terminate the captive core instance. Once stellarCoreRunner.close() is called any pending operations (PrepareRange() or GetLedger()) should return immediately with an error and the ingestion state machine will handle the error by retrying.

• When the retry happens it will create a new instance of stellarCoreRunner which should use the newest stellar core binary.

Why

See #3602 for motiviation.

[Shaptic]

👍

[bartekn]

@tamirms please merge when ready.