configure.ac: fix build with --enable-bufferevents

When building with bufferevents enabled, configure.ac throws an
error if "$ac_cv_header_event2_bufferevent_ssl_h" is not set to
"yes".  However, nowhere was AC_CHECK_HEADERS(event2/bufferevent_ssl.h)
done.  This commit adds the check.
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
changes file for 12474, 12438.