exp/ingest/ledgerbackend: Wrap metaPipe in bufio.Reader

[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

Wraps metaPipe in stellarCoreRunner in bufio.Reader. This improves the speed of reading from a meta pipe.

Why

In 936bc3a we removed bufio.Reader based on an observation that it slows down reading from a pipe. However, after running a CPU profiler on reingestion code to debug another change I realized that a lot of time is spent on reading from a pipe in methods like xdr.Decoder.DecodeInt or xdr.Decoder.DecodeUint (that read just 4 bytes):

When reingesting 1000 ledgers all executions of this method take 17% of entire run time! It was clear that there's an overhead time connected to reading from a pipe. After wrapping meta pipe in bufio.Reader the speed of reingestion of the same range improved by 25% (this isn't affected by Stellar-Core init time, the timer starts after PrepareRange is done).

Before:

INFO[2020-06-30T12:03:44.671+02:00] Reingestion done     duration=74.352344343 from=29999000 pid=44533 service=expingest to=30000000

After:

INFO[2020-06-30T12:06:14.829+02:00] Reingestion done     duration=55.449094814 from=29999000 pid=45181 service=expingest to=30000000

The change in 936bc3a solves the problems with reflection used in XDR decoder being slow. This commit mitigates issues with overhead time added when reading from a pipe.

Known limitations

It's worth checking if there is a similar speed improvement on linux systems (I'm running MacOS), it can be system dependent.

[bartekn]

@2opremio because you have more in-depth info about this, do you want to check before I merge?

[2opremio]

I did, I just didn't approve because Tamir already had.