Replace 'long' with 'int64_t' in public core headers

include/grpc/impl/codegen/time.h

include/grpc/support/avl.h

src/core/iomgr/iocp_windows.c

src/core/iomgr/pollset_posix.c

src/core/support/avl.c

src/core/support/time.c

src/node/ext/timeval.cc

src/python/grpcio/grpc/_cython/imports.generated.c

src/python/grpcio/grpc/_cython/imports.generated.h

src/ruby/ext/grpc/rb_grpc_imports.generated.h

test/core/statistics/census_log_tests.c

test/core/support/avl_test.c

test/core/support/sync_test.c

test/core/support/time_test.c

test/core/util/test_config.h

test/cpp/util/time_test.cc