Use single logging context for everything

Use single logging context for everything

Instead of juggling different logging context, use one single logging
context that gets initialized in the libisc constructor and destroyed in
the libisc destructor.

The application is still responsible for creating the logging
configuration before using the isc_log API.

This patch is first in the series in a way that it is transparent for
the users of the isc_log API as the isc_log_create() and
isc_log_destroy() are now thin shims that emulate the previous
functionality, but it isc_log_create() will always return internal
isc__lctx pointer and isc_log_destroy() will actually not destroy the
internal isc__lctx context.

Signed-off-by: Ondřej Surý <ondrej@isc.org>