crash due to concurrent map read and map write

[fhs]

Full stack trace: crash-20190321-mux9p.txt

fatal error: concurrent map read and map write

goroutine 138 [running]:
runtime.throw(0x6c7984, 0x21)
        /home/big/go/src/runtime/panic.go:617 +0x72 fp=0xc0001c6650 sp=0xc0001c6620 pc=0x42cfb2
runtime.mapaccess2(0x669260, 0xc0004b8600, 0xc0001c66ce, 0xc0001c66e8, 0x46ac67)
        /home/big/go/src/runtime/map.go:472 +0x272 fp=0xc0001c6698 sp=0xc0001c6650 pc=0x40ee52
github.com/fhs/mux9p.(*Config).deleteTag(0xc00009cab0, 0xc0004b8600, 0x2, 0xc0002322c0, 0xb)
        /home/fhs/go/src/github.com/fhs/mux9p/mux9p.go:767 +0x55 fp=0xc0001c6718 sp=0xc0001c6698 pc=0x5e05a5
github.com/fhs/mux9p.(*Config).connoutthread(0xc00009cab0, 0xc00039e230)
        /home/fhs/go/src/github.com/fhs/mux9p/mux9p.go:481 +0x13a fp=0xc0001c67d0 sp=0xc0001c6718 pc=0x5de82a
runtime.goexit()
        /home/big/go/src/runtime/asm_amd64.s:1337 +0x1 fp=0xc0001c67d8 sp=0xc0001c67d0 pc=0x459591
created by github.com/fhs/mux9p.(*Config).listenthread
        /home/fhs/go/src/github.com/fhs/mux9p/mux9p.go:204 +0x79