services/horizon: Trivial improvements to Stellar Core log parsing

[Shaptic]

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

Improves log output:

• Infof -> Info for non-formatted strings
• Lines w/o pubkey prefix are parsed correctly

Why

When the line from Stellar Core's logs contains %, it's interpreted as a formatting directive by the output function (e.g. Infof), causing extraneous output. It should just use Info to avoid this.

Not all Core lines begin with a public key prefix G[AB12] (e.g. ones that begin with <startup> instead), so we shouldn't enforce this in the regex.