add weak symbol _RegisterThriftProtocol in mac
fix the version get in the latest MacOS 10.13+

Removed thrift_binary_head.h and renamed thrift_binary_head_t to thrift_head_t which is in thrift_message.h
Adapt thrift 0.11 which uses shared_ptr defined in stdcxx.h
Simplify example/thrift_extension_c++ by roundrobin between processing by handler or directly.
Removed libbrpc_thrift.a, the objs are archived into libbrpc.a (and libbrpc.so) directly

Add thrift protocol for brpc.

mac docs update

macos support

+ Remove unnecessary loading of openssl.cnf

Update threading_overview.md