src/or/connection.c: expose bucket_millis_empty for bufferevents test

Currently tor fails to build its test when enabled with bufferevents
because an #ifndef USE_BUFFEREVENTS hides bucket_millis_empty() and
friends.  This is fine if we don't run tests, but if we do, we need
these functions in src/or/libtor-testing.a when linking src/test/test.

This patch moves the functions outside the #ifndef and exposes them.

See downstream bug:

	https://bugs.gentoo.org/show_bug.cgi?id=510124