rgw: dump remaining coroutines when cr deadlock is detected

[cbodley]

when we detect a coroutine deadlock, dump remaining coroutines to lderr before asserting. this should help us figure out what's causing the deadlocks in teuthology runs. example output:

2016-10-20 12:17:28.236470 7f52959dea80 -1 run(): ERROR: deadlock detected, dumping remaining coroutines:
[
    {
        "stack": "0x55f9f6bdc3d0",
        "run_count": 0,
        "ops": [
            {
                "type": "17RGWFetchAllMetaCR",
                "spawned": [
                    "0x55f9f6bde3c0"
                ],
                "status": {
                    "status": "acquiring lock",
                    "timestamp": "2016-10-20 16:17:28.235549Z"
                }
            }
        ]
    },
    {
        "stack": "0x55f9f6bde3c0",
        "run_count": 0,
        "ops": [
            {
                "type": "11MockLeaseCR"
            }
        ]
    }
]

[yehudasa]

@cbodley shouldn't we use _derr?

[cbodley]

this is referring to the std::ostream* _dout variable from the dout_impl macro in common/dout.h, which the other dout/derr macros are using

[cbodley]

i.e. it's the stream associated with the lderr() on the line above, and closed with dendl on the line below