As per the Go Wiki page on cgo [1] passing callback with data between C
and Go is non-trivial due to the pointer passing rules [2]. This change
creates a new helper type, based on the examples on the wiki, that uses
opaque indexes to exchange between C and Go.
For now, this type is simple as we're only going to have a few callbacks
initially but this could easily be improved and made more general in the
future as needed. Because we anticipate having to use this type in all
of our core packages (rados and cephfs as well as rbd) we are creating
the type in an "internal" package so that we don't treat the new type
as public despite the capitalized method names.
[1]: https://github.com/golang/go/wiki/cgo
[2]: https://golang.org/cmd/cgo/#hdr-Passing_pointers
Signed-off-by: John Mulligan <jmulligan@redhat.com>
