Better poller

Closes #12 (closed)

Closes #13 (closed)

Don't review:

• pktline package was copied from Gitaly https://gitlab.com/gitlab-org/gitaly/-/tree/master/internal/git/pktline
• reference_discovery.go and reference_discovery_test.go was copied from Gitaly https://gitlab.com/gitlab-org/gitaly/-/tree/master/internal/git/stats
• Updated generated files - BUILD.bazel and various mocks.

I'm planning to add an RPC to Gitaly to allow fetching parsed ref advertisement and then use it instead of the copied code. More convenient API gitlab-org/gitaly!2510 (closed). This MR is needed to improve the current situation and then it'll be trivial to replace the code with the Gitaly call in a later MR because polling is now happening in a separate Poller object and is decoupled from it's usage.