Change name of thread_pool files to fixed_size_thread_pool when

BUILD

Makefile

build.json

include/grpc++/fixed_size_thread_pool.h

src/cpp/server/create_default_thread_pool.cc

src/cpp/server/fixed_size_thread_pool.cc

src/cpp/server/server_builder.cc

test/cpp/end2end/client_crash_test.cc

test/cpp/end2end/end2end_test.cc

test/cpp/end2end/mock_test.cc

test/cpp/end2end/server_crash_test.cc

test/cpp/end2end/thread_stress_test.cc

test/cpp/qps/server_async.cc

test/cpp/qps/server_sync.cc

test/cpp/server/fixed_size_thread_pool_test.cc

test/cpp/util/cli_call_test.cc

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/run_tests/sources_and_headers.json

tools/run_tests/tests.json