libceph: rename and export con->state states

In preparation for msgr2, rename msgr1 specific states and move the
defines to the header file.

Also drop state transition comments.  They don't cover all possible
transitions (e.g. NEGOTIATING -> STANDBY, etc) and currently do more
harm than good.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>