Issue: #9261

Before this PR, every C++ CompletionQueue object creation ended up calling grpc_init which acquires a mutex g_init_mu to increment the number of grpc-init invocations.

This is ok if the number of completion queues being created was small. However, in case of C++ Synchronous server, a completion queue is create for every call. This ends up making g_init_mu one of top created mutexes in sync servers.

This PR makes calling grpc_init optional when creating C++ CompletionQueue objects. It is always set to true by default but if a C++ CompletionQueue is being created by passing a grpc_completion_queue * pointer i.e the following constructor (which is what Sync C++ server calls when creating a per-call completion queue), it is okay to NOT call grpc_init.

class CompletionQueue : private GrpcLibraryCodegen {
 ..
  explicit CompletionQueue(grpc_completion_queue* take);
..
}

This change is safe because if we got a grpc_completion_queue * structure, we MUST have already called grpc_init().

Benchmark results
I added a new benchmark BM_CreateDestroyCpp2 (didn't previously include it as a part of the PR because I thought it is trivial - but on a second thought, i think it is a good idea :))

Locks are down from 3 to 1

On master (with BM_CreateDestroyCpp2 temporarily ported):

sreek@sreek-dev:~/workspace/grpc (master) $ bins/counters/bm_cq --benchmark_filter=BM_CreateDestroyCpp2
Run on (12 X 3501 MHz CPU s)
2017-03-29 10:54:34
Benchmark                     Time           CPU Iterations
-----------------------------------------------------------
BM_CreateDestroyCpp2        215 ns        215 ns    3230778 locks/iter:3 atm_cas/iter:0 atm_add/iter:3 allocs/iter:1

On this PR branch:

sreek@sreek-dev:~/workspace/grpc (init-free-cq) $ bins/counters/bm_cq --benchmark_filter=BM_CreateDestroyCpp2
Run on (12 X 3501 MHz CPU s)
2017-03-29 10:56:15
Benchmark                     Time           CPU Iterations
-----------------------------------------------------------
BM_CreateDestroyCpp2        160 ns        160 ns    4413813 locks/iter:1 atm_cas/iter:0 atm_add/iter:3 allocs/iter:1