completion-io: crash on async destruction with pending writes

(lldb) bt    

* thread #1: tid = 0, 0x00007f623f2905b9 couchbase_impl.node`on_flush_done(ctx=0x0000000004076720, expected=2543, actual=2543) + 56 at mcserver.c:84, name = 'node', stop reason = signal SIGSEGV

  * frame #0: 0x00007f623f2905b9 couchbase_impl.node`on_flush_done(ctx=0x0000000004076720, expected=2543, actual=2543) + 56 at mcserver.c:84

    frame #1: 0x00007f623f2cfce8 couchbase_impl.node`Cw_ex_handler(sd=0x0000000004019ca0, status=0, wdata=0x00000000000009ef) + 68 at ctx.c:527

    frame #2: 0x00007f623f28fbb5 couchbase_impl.node`write2_callback(req=0x0000000004035e30, status=0) + 95 at plugin-libuv.c:383

    frame #3: 0x0000000001b0f9e4 node`uv__write_callbacks(stream=0x0000000004019cf8) + 328 at stream.c:940

    frame #4: 0x0000000001b0e7e9 node`uv__stream_destroy(stream=0x0000000004019cf8) + 312 at stream.c:450

    frame #5: 0x0000000001b019f5 node`uv__finish_close(handle=0x0000000004019cf8) + 145 at core.c:256

    frame #6: 0x0000000001b01af9 node`uv__run_closing_handles(loop=0x000000000285c3e0) + 68 at core.c:286

    frame #7: 0x0000000001b01d17 node`uv_run(loop=0x000000000285c3e0, mode=UV_RUN_ONCE) + 202 at core.c:356

    frame #8: 0x0000000001953fec node`node::StartNodeInstance(arg=0x00007ffd822ccbe0) + 727 at node.cc:4394

    frame #9: 0x00000000019542f1 node`node::Start(argc=2, argv=0x0000000003f87bc0) + 241 at node.cc:4474

    frame #10: 0x000000000198918e node`main(argc=3, argv=0x00007ffd822ccd48) + 62 at node_main.cc:53

    frame #11: 0x00007f6241578b45 libc.so.6`__libc_start_main + 245

    frame #12: 0x0000000000ded529 node

(lldb) f 0

frame #0: 0x00007f623f2905b9 couchbase_impl.node`on_flush_done(ctx=0x0000000004076720, expected=2543, actual=2543) + 56 at mcserver.c:84

   81  	    mc_SERVER *server = lcbio_ctx_data(ctx);

   82  	    lcb_U64 now = 0;

   83  	

-> 84  	    if (LCBT_SETTING(server->instance, readj_ts_wait)) {

   85  	        now = gethrtime();

   86  	    }

   87