Eliminate public thread-pool interface

BUILD

Makefile

build.json

include/grpc++/server_builder.h

src/cpp/server/create_default_thread_pool.cc

src/cpp/server/dynamic_thread_pool.cc

src/cpp/server/dynamic_thread_pool.h

src/cpp/server/fixed_size_thread_pool.cc

src/cpp/server/fixed_size_thread_pool.h

src/cpp/server/server.cc

src/cpp/server/server_builder.cc

src/cpp/server/thread_pool_interface.h

test/cpp/end2end/end2end_test.cc

test/cpp/end2end/mock_test.cc

test/cpp/end2end/thread_stress_test.cc

test/cpp/qps/server_sync.cc

test/cpp/server/dynamic_thread_pool_test.cc

test/cpp/server/fixed_size_thread_pool_test.cc

test/cpp/util/cli_call_test.cc

tools/doxygen/Doxyfile.c++