core: basic xattr support

Adding get and set xattr support.

Signed-off-by: Adam Manzanares <nmtadam@gmail.com>
Reviewed-by: Noah Watkins <noahwatkins@gmail.com>
test: add xattr error test cases

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
core: add support for IoCtx::exec

Passes internal tests, so should be ready to go. There isn't a good test
target in default Ceph installation to use for unit test yet.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge branch 'wip-exec'

Conflicts:
	lua_rados.cc
	test.lua

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>