librbd: remove mostly-useless assign_bid helper

Do it inline.

Signed-off-by: Sage Weil <sage@inktank.com>
cls_rbd: remove old assign_bid method

This method is problematic because it both writes/mutates and returns data,
which means that an untimely client disconnect or peering event will result
in a success to the client with no payload.

It has not been used since v0.52 (18054ba46fe2779d8df8b1a0d69ec93ca6a66c34)
which is pre-bobtail; so this change breaks compatibility with pre-bobtail
librbd clients (at least for image creation).

Signed-off-by: Sage Weil <sage@inktank.com>
cls/hello: hello, world rados class

Simple example of a rados class doing read, write, and read/modify/write
methods.

Signed-off-by: Sage Weil <sage@inktank.com>