horizon/ingest: refactor initial load of orderbook offers/pools to use functional streaming

[sreuland]

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

at init time, entire orderbook was loaded into intermediate ram(slice) first from db resultset, then the slice was copied piece by piece into the orderbook memory, doubling the overall ram size needed by orderbook state for some initial amount of time.

Why

performance improvement to load orderbook data more efficiently at startup, avoid the double ram usage, handle larger sizes as things scale up

Closes #1622

Known limitations

does not change final orderbook state, it is still stored in ram

[bartekn]

I remember one issue in our XDR decoder: an object reused for decoding was not cleared so sometimes pointer fields contained the old data. I think this can be also the case here. Can we check? The fix would be to move liquidityPool variable inside the for loop.

[bartekn]

I found the PRs for reference: stellar/go-xdr#13 stellar/go-xdr#14.

[sreuland]

In this case, every field in the struct is set to a column value in results(selected from liquidity_pools table) per row iteration during StructScan, so, shouldn't need init between iterations.