osd: add infrastructure to block io on an obc

Add an is_blocked() method for the obc, and add infrastructure to block
any operations of it returns true.  Clean up on_change(), and add a helper
to kick an obc when whatever condition leading to it being blocked is no
longer true.

For now, is_blocked() is always false...

Signed-off-by: Sage Weil <sage@inktank.com>
osd: block requests on object during COPY_FROM

Block any request on an object (read or write) during the COPY_FROM
operation.

This could potentially be broken down into read vs write operations without
much difficulty, but blocking any op indescriminately is sufficient for
now, so let's keep it simple.

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