Fix#9001
The GZIP ProxyReponseWriter doesn't currently respond correctly
to requests about its Written status - leading to #9001.
This PR properly reimplements these methods.
* 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.
