Provide guaranty that Close forces the driver to stop sending to watchq
and to close it. See a5dbb92b ("go/zodb: Require drivers to close watchq
on Close") for details.

Without the fix TestWatch fails with test timeout:

    panic: test timed out after 30s

    # Close waits for serve to stop
    goroutine 93 [semacquire]:
    sync.runtime_Semacquire(0xc000152170)
            /home/kirr/src/tools/go/go/src/runtime/sema.go:56 +0x45
    sync.(*WaitGroup).Wait(0xc000152168)
            /home/kirr/src/tools/go/go/src/sync/waitgroup.go:130 +0x65
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).Close(0xc0001520f0, 0x1313, 0x1)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:159 +0x47
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zeo).Close(0xc000313680, 0xc000107c78, 0x1)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo.go:526 +0x2e
    lab.nexedi.com/kirr/neo/go/internal/xtesting.DrvTestWatch(0xc000082c00, 0xc0000aa2a0, 0x24, 0x6a4a38)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/internal/xtesting/xtesting.go:442 +0xdb5
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.TestWatch.func1(0xc000082c00, 0x6e3498, 0xc00009a380)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:270 +0x99
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func2.1(0xc0000a4168, 0x16)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:207 +0xfb
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func1(0xc000082c00, 0xc00009c180)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:186 +0x129
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func2(0xc000082c00)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:199 +0x10e
    testing.tRunner(0xc000082c00, 0xc00009c160)
            /home/kirr/src/tools/go/go/src/testing/testing.go:1194 +0xef
    created by testing.(*T).Run
            /home/kirr/src/tools/go/go/src/testing/testing.go:1239 +0x2b3

    # serve is stuck in invalidateTransaction doing watchq<-
    goroutine 26 [chan send]:
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zeo).invalidateTransaction(0xc000313680, 0x6417e0, 0xc000323b60, 0x0, 0x0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo.go:176 +0x373
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).serveRecv1(0xc0001520f0, 0xc000393890, 0x0, 0x0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:225 +0x4b4
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).serveRecv(0xc0001520f0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:176 +0x8d
    created by lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).start
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:99 +0xc8