mainloop: Rename global_write_bucket_low()

That function is only used to test the global bucket write limit for a
directory connection.

It should _not_ be used for anything else since that function looks to see if
we are a directory authority.

Rename it to something more meaningful. No change in behavior at this commit,
only renaming.

Part of #33029

Signed-off-by: David Goulet <dgoulet@torproject.org>