Improve Linking and Unlink object pools RPC

Closes #1423 and #1420

See merge request gitlab-org/gitaly!1000

GitLab-Rails performs operations when Linking and Unlinking. This makes sure
we perform less RPCs from Rails, and keeps the responsibilities straight. To do
so, config options needed to be set. An RPC was available to set it
through Ruby, but proved quite cumbersome to use as the rubyserver now
needed to go into the `objectpool` package.

Fixes: gitlab-org/gitaly#1420

As a matter of applying the boyscout rule, this commit also sets the
gc.auto option for object pools to 0. This means that Git will never
perform GC without being told to do so.

Fixes: gitlab-org/gitaly#1423

Fix tests failing due to test-repo change

Closes #1434

See merge request gitlab-org/gitaly!1004

Update parser gem to get rid of warnings

See merge request gitlab-org/gitaly!1002

The previous version would spew these warnings:

```
warning: parser/current is loading parser/ruby25, which recognizes
warning: 2.5.1-compliant syntax, but you are running 2.5.3.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.
```

Add advice about how to approach Gitaly changes

See merge request gitlab-org/gitaly!956

Backport changelog of 1.7.1 to master

See merge request gitlab-org/gitaly!999

Add explicit null byte check in internal/command.New

See merge request gitlab-org/gitaly!997

Log correlation_id field in structured logging output

See merge request gitlab-org/gitaly!995

README cleanup

See merge request gitlab-org/gitaly!996

Clean up a repository by applying a BFG object map to it

See merge request gitlab-org/gitaly!990

Clean up invalid keep-around refs when performing housekeeping

See merge request gitlab-org/gitaly!992

We used to corrupt `keep-around` refs when creating them using
rugged. This ensures we clean up any refs git won't understand
repairing them if possible.

When the ref contents is blank, invalid or a "blank"-sha, we remove
the ref and recreate it based on it's filename if the filename was a
valid SHA.

This is run as part of the `GarbageCollect` RPC

Fix TLS client code on macOS

See merge request gitlab-org/gitaly!994

Add tls configuration to gitaly golang server

See merge request gitlab-org/gitaly!932

Update to latest goimports formatting

See merge request gitlab-org/gitaly!993

Link and Unlink RPCs

Closes #1345 and #1349

See merge request gitlab-org/gitaly!986

To use an object pool, each repository needs to be able to link and
unlink itself. This is done through writing the alternates file
manually.

To the best of my knowledge there's no command on git to perform this.

Simplify versioning process

See merge request gitlab-org/gitaly!989