librados: add cmpext API

librados: add cmpext API
The compare-extent (cmpext) operation allows callers to compare existing
object contents with an arbitrary buffer. cmpext requests can be
compounded with read and write operations, allowing for atomic object
content updates. return 0 on success, negative error code
on failure, (-MAX_ERRNO - mismatch_off) on mismatch

This commit is based on Mike Christie's initial C++ API, with the
addition of AIO support and a C API. Response marshalling was also
reworked, so that the miscompare offset is unmarshalled transparently to
the caller.

Signed-off-by: Zhengyong Wang <wangzhengyong@cmss.chinamobile.com>
Signed-off-by: David Disseldorp <ddiss@suse.de>