hooks: Convert GL_ values to be injected via the hooks payload
This is the final piece in converting the set of environment variables we had previously to instead use the hooks payload by moving GL_ values into the payload. With this commit, hooks only need to variables to be set: GITALY_BIN_PATH to locate the gitaly-hooks binary, and then GITALY_HOOKS_PAYLOAD which carries all required metainformation.
As before, I've added fallback mechanisms to enable zero-downtime upgrades. The order of commits also demonstrates that this fallback works as expected.
Fixes #3201 (closed)