conf: add observer framework

conf: add observer framework

Objects can now register as configuration observers interested in a
subset of the configuration keys. The observers will be told exactly
which keys have changed.

The first user is dout, which now no longer needs the infamous SIGHUP
hack to know when to reopen the config file.

librados: Remove rados_reopen_log, which was basically a means for the
library user to trigger the SIGHUP behavior.

Changes are accumulated and applied all at once by apply_changes. This
function is called as part of common_init, and after every call to
injectargs.

Signed-off-by: Colin McCabe <colin.mccabe@dreamhost.com>