[#348] fix crash on early-recv error
[#348] fix crash on early-recv error

* The SendStream.Close() was not called by dataconn.Server, which left the zfs send process dangling.
* When the source job's ctx interceptor closed the task, the dangling zfs send was detect by the trace package and panicked.

    020-07-25T19:54:41-04:00 [ERRO][latitude][rpc.data][cyZj$J3Ca$J3Ca.CJwB]: cannot write send stream err="frameconn: shutting down"
    panic: end task: 1 active child tasks: end task: task still has active child tasks

    goroutine 196966 [running]:
    github.com/zrepl/zrepl/daemon/logging/trace.WithTask.func1.1(0xc000320680, 0xcde000)
            /home/jeremy/go/src/github.com/zrepl/zrepl/daemon/logging/trace/trace.go:221 +0x2f7
    github.com/zrepl/zrepl/daemon/logging/trace.WithTask.func1()
            /home/jeremy/go/src/github.com/zrepl/zrepl/daemon/logging/trace/trace.go:237 +0x38
    github.com/zrepl/zrepl/daemon/logging/trace.WithTaskAndSpan.func1()
            /home/jeremy/go/src/github.com/zrepl/zrepl/daemon/logging/trace/trace_convenience.go:41 +0x37
    github.com/zrepl/zrepl/daemon/job.(*PassiveSide).Run.func1(0xdcf780, 0xc0000a3560, 0xdc65a0, 0xc00035e620, 0xc0000a34d0)
            /home/jeremy/go/src/github.com/zrepl/zrepl/daemon/job/passive.go:194 +0x2e7
    github.com/zrepl/zrepl/rpc.NewServer.func3(0xdcf780, 0xc0001ce4b0, 0xdc65e0, 0xc00035e600, 0xc0000a34d0)
            /home/jeremy/go/src/github.com/zrepl/zrepl/rpc/rpc_server.go:82 +0xd5
    github.com/zrepl/zrepl/rpc/dataconn.(*Server).serveConn(0xc0000a2ba0, 0xc00018eca0)
            /home/jeremy/go/src/github.com/zrepl/zrepl/rpc/dataconn/dataconn_server.go:149 +0x3be
    github.com/zrepl/zrepl/rpc/dataconn.(*Server).Serve.func3(0xc0000b8180, 0xc0000a2ba0, 0xc00018eca0)
            /home/jeremy/go/src/github.com/zrepl/zrepl/rpc/dataconn/dataconn_server.go:108 +0x5d
    created by github.com/zrepl/zrepl/rpc/dataconn.(*Server).Serve
            /home/jeremy/go/src/github.com/zrepl/zrepl/rpc/dataconn/dataconn_server.go:106 +0x24a
    2020-07-25T19:58:55-04:00 [ERRO][latitude][rpc.data][Pt4F$gCWT$gCWT.fzhc]: cannot write send stream err="frameconn: shutting down"
    panic: end task: 1 active child tasks: end task: task still has active child tasks

fixes #348

zrepl/zrepl,zrepl/zrepl