common/RefCountedObject: optionally debug

Introduce a 'refs' subsys for logging.  Print log ref count inc and dec
for any RefCountedObject that gets a cct passed to its ctor.

Signed-off-by: Sage Weil <sage@inktank.com>
common/RefCountedObject: optionally take initial ref count to ctor

Signed-off-by: Sage Weil <sage@inktank.com>
common/RefCountedObject: assert nref == 0 in dtor

Signed-off-by: Sage Weil <sage@inktank.com>
osd: debug Session refs

Signed-off-by: Sage Weil <sage@inktank.com>
msg: debug Connection refs

Signed-off-by: Sage Weil <sage@inktank.com>
msg/Pipe: debug Pipe refs

Signed-off-by: Sage Weil <sage@inktank.com>
msg/Message: use RefCountedObject ctor to set initial ref count

Also, drop the useless assert in the dtor (the RefCountedObject dtor does
the same).

Signed-off-by: Sage Weil <sage@inktank.com>
common/RefCountedObject: make nref, cct private

Signed-off-by: Sage Weil <sage@inktank.com>
msg: debug refs on incoming Messages

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #2058 from ceph/wip-refs

refcount debugging for RefCountedObject

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
doc: Fix a typo regarding requiretty for RHEL based platforms