* Pooled and buffered gzip implementation

* Add test for gzip

* Add integration test

* Ensure lfs check within transaction

The previous code made it possible for a race condition to occur whereby a LFSMetaObject could be checked into the database twice. We should check if the LFSMetaObject is within the database and insert it if not in one transaction.

* Try to avoid primary key problem in postgres

The integration tests are being affected by
https://github.com/go-testfixtures/testfixtures/issues/39 if we set the
primary key high enough, keep a count of this and remove at the end of
each test we shouldn't be affected by this.