Add CephContext

Add CephContext

A CephContext represents the context held by a single library user.
There can be multiple CephContexts in the same process.

For daemons and utility programs, there will be only one CephContext.
The CephContext contains the configuration, the dout object, and
anything else that you might want to pass to libcommon with every
function call.

Move some non-config things out of md_config_t and into CephContext.

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