Prevent blocking custom hook output streams

Based on https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/49

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/1458

See merge request !67

Put Redis loading code in one place

Before this change you could not run the gitlab_net specs in isolation.

See merge request !64

ARGF does more than we want

ARGF is a virtual IO stream that treats the elements of ARGV as paths
to files to be opened. In practice, git-receive-pack will probably
never pass arguments to pre-receive and post-receive because it is not
supposed to (according to 'man 5 githooks'). In case it does, however
our hooks should not have the surprise behavior of treating those
arguments as files to be opened.

See merge request !66

ARGF is a virtual IO stream that treats the elements of ARGV as paths
to files to be opened. In practice, git-receive-pack will probably
never pass arguments to pre-receive and post-receive because it is not
supposed to (according to 'man 5 githooks'). In case it does, however
our hooks should not have the surprise behavior of treating those
arguments as files to be opened.

Simplify the GitAccessStatus class

Make the optional 'message' argument required. Remove unused 'to_json'
method.

See merge request !65

Make the optional 'message' argument required. Remove unused 'to_json'
method.

Update PostReceive worker to provide enqueued_at time

See merge request !63