Refactor FsAdaptor filesystem monitors to monitor multiple startpoints.

In order to support the changes for the following bugs:

b/6513820: Enumerate share points or Namespaces given a hostname
b/12897821: Enumerate Namespace target links

Supplying only hostname or DFS Namespace as a startpoint
could eventually resolve to multiple shares being traversed.
In anticipation of these changes, this refactoring alters the
WindowsFileDelegate to allow multiple file system monitors.
To simplify this change, this refactoring also removes the
unused stopMonitorPath method from the FileDelegate interface.