[FIX] fix compile on ubuntu18.04 + use anonymous namespace for hardco…

[Simonhancrew]

Problem Summary:

1. comile error on ubuntu18.04, BRPC/src/brpc/redis_command.cpp:411:29: error: ‘numeric_limits’ is not a member of ‘std’
2. CTX_WIDTH should in anonymous namespace, only used in this cpp file.

Changed:

1. compile on ubuntu18.04, redis_commands.cpp use numeric_limits<>, while missing header, cause compile error
2. use anonymous namespace for hardcode var, dont emit this to brpc namespace

Side effects:

• no side effect

[Simonhancrew]

@wwbmmm 烦请老哥take a look, 谢谢