proto: optimize global (un)marshal lock using RWMutex

[TennyZhuang]

This PR is also submitted to golang/protobuf#1004

This is a workaround to #656

Signed-off-by: TennyZhuang zty0826@gmail.com

Thie PR Use RWMutex to optimize getMarshalInfo and getUnmarshalInfo, for these functions, only n (number of message type) will hit Write, and m(number of message) - n will hit Read, it's the best case to use RWMutex instead of Mutex.

This optimization introduce huge improvement in our scenario.

We have 1000 worker and 1 controller, and work and controller keep heartbeat with gRPC. They also exchange the job info with each other.

The message is like

message Job {
    uint64 job_id = 1;
    // Some other info
}

message Heartbeat {
    repeated Job jobs = 1;
}

About 10000 jobs in every Heartbeat, and the heartbeat QPS in controller is about 1000.

The controller handle the Heartbeat in about 10ms, and the network latency is about 10ms, but the client will use about 30s in maximum to finish a RPC call.

We use golang pprof block profile, and it seems that almost all block is caused by one global Mutex in protobuf package.

After optimization, in our use case, the rpc call from client will only use about 30ms, as our expected.