services/horizon: Ignore ConnectionTimeout in txsub

[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 changes the behaviour of --connection-timeout config param. Right now, it is ignored in txsub so if the value is less than 30s, txsub will still timeout after 30s.

Why

If Horizon admin wants to set --connection-timeout to a small value it will likely affect txsub system because it requires at least 5 seconds on average to report the status of the transaction when it's included in the ledger.

Known limitations

Currently the code ignores timeout for all POST requests to avoid having to parse the route from request URL.

[abuiles]

👍