[VFS-694] Fix inability to start the DefaultFileMonitor after it has been stopped.
[VFS-694] Fix inability to start the DefaultFileMonitor after it has
been stopped.

Closes #55.
seeburger-ag/commons-vfs,seeburger-ag/commons-vfs,seeburger-ag/commons-vfs,apache/commons-vfs,ecki/commons-vfs,apache/commons-vfs,ecki/commons-vfs