MB-11859: 'connections' stats returns... JSONified

Restore the 'connections' stats removed by 7aaf80e, bringing back in
JSON format for richer formatting / ease of manipulation. Each
connection structure is returned as a 'stat' document, with the value
a JSON document describing that conneciton.

Example (for a single connection):

    {
      "aiostat": 0,
      "cas": "0x0",
      "cmd": 16,
      "conn": "0x10be9c900",
      "dynamic_buffer": {
        "buffer": "0x0",
        "offset": 0,
        "size": 0
      },
      "engine_storage": "0x10968a160",
      "ev_flags": 18,
      "ewouldblock": false,
      "iov": {
        "ptr": "0x10bce8080",
        "size": 10,
        "used": 0
      },
      "item": "0x0",
      "itemlist": {
        "curr": "0x0",
        "left": 0,
        "list": "0x0",
        "size": 0
      },
      "keylen": 11,
      "list_state": 0,
      "msglist": {
        "bytes": 0,
        "curr": 0,
        "list": "0x1041d21c0",
        "size": 5,
        "used": 1
      },
      "nevents": 20,
      "next": "0x0",
      "noreply": false,
      "opaque": "0xffffffffb46231bf",
      "protocol": "binary",
      "read": {
        "buf": "0x10423b000",
        "bytes": 0,
        "curr": "0x10423b023",
        "size": 2048
      },
      "refcount": 1,
      "registered_in_libevent": true,
      "ritem": "0x10423b023",
      "rlbytes": 0,
      "sbytes": 0,
      "socket": 47,
      "state": [
        "conn_nread",
        "bin_reading_packet"
      ],
      "store_op": 0,
      "tap_iterator": "0x0",
      "temp_alloc_list": {
        "curr": "0x107191720",
        "left": 0,
        "list": "0x107191720",
        "size": 20
      },
      "thread": "0x104216270",
      "transport": "TCP",
      "which": 2,
      "write": {
        "buf": "0x109980000",
        "bytes": 0,
        "curr": "0x109980000",
        "size": 2048
      },
      "write_and_free": "0x0",
      "write_and_go": "0x1032bc3d0"
    }

Change-Id: I8f955c550d6968846ed4919af1761e95b603537f
Reviewed-on: http://review.couchbase.org/40924
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>