move a bunch of reader state fields into 'connection'
These fields are constant, seldom read, and associated with the
connection.
Also, move #connection definition from rabbit.hrl to rabbit_reader -
nothing else is using this and it is really private to the reader.
move some more reader state into #connection
and also clear auth_state once auth has been completed, so we don't
hang on to some potentially large piece of state.
move throttle related reader state into sub-state
Merge bug25368