autotools: use -Wall for the module build

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
core: use retval helpers and update docs

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
core: track conn state to prevent API misuse

Tracks state of cluster object, and prevents misuse of the RADOS API
that would result in segfaults and other bad things, and instead throw
a Lua error and with an error message.

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