support/compressxdr: Fix compressxdr performance

[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 reviewed the changes in this PR and if I consider them worthwhile for being mentioned on release notes then I have updated the relevant CHANGELOG.md within the component folder structure. For example, if I changed horizon, then I updated (services/horizon/CHANGELOG.md. I add a new line item describing the change and reference to this PR. If I don't update a CHANGELOG, I acknowledge this PR's change may not be mentioned in future release notes.
• 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

In the initial ledger exporter prototype I suggested using github.com/stellar/go-xdr/xdr3 to encode / decode XDR blobs. This was actually a mistake because we should be using the optimized encode / decode functions in github.com/stellar/go/xdr. This commit corrects that mistake and consequently improves decode performance by 6-7x and encode performance by 4-5x.

Before:

BenchmarkDecodeLedgerCloseMetaBatch
BenchmarkDecodeLedgerCloseMetaBatch-10 100 11628515 ns/op
PASS

After:

BenchmarkDecodeLedgerCloseMetaBatch
BenchmarkDecodeLedgerCloseMetaBatch-10 658 1821993 ns/op
PASS

Before:

BenchmarkEncodeLedgerCloseMetaBatch
BenchmarkEncodeLedgerCloseMetaBatch-10 133 8959840 ns/op
PASS

After:

BenchmarkEncodeLedgerCloseMetaBatch
BenchmarkEncodeLedgerCloseMetaBatch-10 573 2104398 ns/op
PASS