readdcw: Fix unwatching at the same time as incoming event

[imsodin]

Scenario:
Stop a watch at the same time as an event is created (in original case: file removed). Then Watch it again. After that no more events are generated and calling Stop again returns an error.

I added some debug statements (see diff). In the following log the first time Stop works fine, the second time it fails and then no more events are registered even though files are created and Stop fails.

2017/08/27 14:35:08.535964 [D] Stop(0xc04203aea0) error: <nil>
2017/08/27 14:35:08.535964 [D] loopstate: stateUnwatch
2017/08/27 14:35:09.950001 [D] dispatching notify.FileActionRenamedOldName on "C:\\Temp\\syncthing-d3d5715d\\src\\github.com\\syncthing\\syncthing\\lib\\fswatcher\\temporary_test_fswatcher\\oldfile"
2017/08/27 14:35:09.950001 [D] dispatching notify.FileActionRenamedNewName on "C:\\Temp\\syncthing-d3d5715d\\src\\github.com\\syncthing\\syncthing\\lib\\fswatcher\\temporary_test_fswatcher\\newfile"
2017/08/27 14:35:13.651173 [D] Stop(0xc04203b500) error: <nil>
2017/08/27 14:35:13.653176 [D] dispatching notify.FileActionRemoved on "C:\\Temp\\syncthing-d3d5715d\\src\\github.com\\syncthing\\syncthing\\lib\\fswatcher\\temporary_test_fswatcher\\newfile"
2017/08/27 14:35:16.366990 [D] Stop(0xc04203b980) error: notify: another re/unwatching operation in progress

I already found a fix, but would like to see whether the test can reproduce the error first.

[imsodin]

As I was suspecting I cannot reproduce this, it probably depends heavily on the host system. I will close this and open a new PR with my proposed fix.