feat:client 对象提供写数据接口

[JerryZhou343]

client 对象已经含有session管理；所以上层使用的client的时候，没必要重复进行session管理，client提供write接口，选择活跃的session写数据即可。

测试示例： tcp-echo client:

func echo() {
    var pkg EchoPackage
    pkg.H.Magic = echoPkgMagic
    pkg.H.LogID = (uint32)(src.Int63())
    pkg.H.Sequence = atomic.AddUint32(&reqID, 1)
    // pkg.H.ServiceID = 0
    pkg.H.Command = echoCmd
    pkg.B = conf.EchoString
    pkg.H.Len = (uint16)(len(pkg.B)) + 1
    /*
    if session := client.selectSession(); session != nil {
        err := session.WritePkg(&pkg, WritePkgTimeout)
        if err != nil {
            log.Warn("session.WritePkg(session{%s}, pkg{%s}) = error{%v}", session.Stat(), pkg, err)
            session.Close()
            client.removeSession(session)
        }
    }
     */
    if err := client.gettyClient.WritePkg(&pkg,WritePkgTimeout);err != nil{
        log.Warn("session.WritePkg(pkg{%s}) = error{%v}",  pkg, err)
    }
}

[codecov-io]

Codecov Report

Merging #50 (f99e888) into master (9f44d78) will decrease coverage by 0.06%. The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   60.32%   60.25%   -0.07%     
==========================================
  Files          20       20              
  Lines        2936     2956      +20     
==========================================
+ Hits         1771     1781      +10     
- Misses        874      883       +9     
- Partials      291      292       +1     

Impacted Files	Coverage Δ
transport/server.go	65.99% <0.00%> (-0.82%)	:arrow_down:
transport/client.go	66.94% <47.05%> (-1.55%)	:arrow_down:
transport/connection.go	79.70% <0.00%> (+0.73%)	:arrow_up:

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