Fixed intermittent CPP sync server shutdown leak.

[arjunroy]

Specifically: if a request handling thread is in flight but scheduled
out when shutdown is called on the server, but it has already passed
the shutdown check, then when it resumes it will add a grpc_call to
the completion queue that is leaked. We fix this by explicitly freeing
such calls after all worker threads have shutdown.

To manifest the leak, run the end2end::ClientCancelsRequestStream
test repeatedly on the unpatched server implementation. About 0.5% of
the time, the leak will manifest.

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[grpc-testing]

****************************************************************

libgrpc.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]


****************************************************************

libgrpc++.so

     VM SIZE                                                                                            FILE SIZE
 ++++++++++++++ GROWING                                                                              ++++++++++++++
  +0.1%     +65 src/cpp/server/server_cc.cc                                                              +65  +0.1%
      [NEW]    +200 void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >    +200  [NEW]
      +9.3%     +80 grpc::Server::ShutdownInternal                                                           +80  +9.3%
       +53%     +73 grpc::Server::SyncRequestThreadManager::Wait                                             +73   +53%
       +14%      +8 grpc::Server::SyncRequest::FinalizeResult                                                 +8   +14%

 -------------- SHRINKING                                                                            --------------
  -0.0%      -5 [None]                                                                                  -993  -0.0%

  +0.0%     +60 TOTAL                                                                                   -928  -0.0%

[grpc-testing]

[trickle] No significant performance differences

[yang-g]

Thanks for the fix.

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 2,019,390      Total (=)      2,019,390

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
11,180,033      Total (>)     11,180,032

 No significant differences in binary sizes

[grpc-testing]

Corrupt JSON data (indicates timeout or crash): 
    bm_call_create.BM_IsolatedFilter_ClientChannelFilter_NoOp_.counters.new: 10
    bm_call_create.BM_IsolatedFilter_ClientChannelFilter_NoOp_.counters.old: 10


[microbenchmarks] No significant performance differences

[arjunroy]

I signed it.

[yang-g]

Thanks!