There are certain operations that aren't considered safe on all NFS

paths. These often involve operations where file locking is required

or the oepration is sensitive to proper synchronization. In these cases,

it can be helpful or important to log some information to help

administrators if something were to act up. This is going to be true for

the local broker.
This change introduces reviewboard.admin.server.is_nfs_path() to help

perform this check. This takes a path and checks to see if there's a

known mount point that it may live in. This is a best-effort check and

is only capable of checking Linux's /proc/mounts. On other sytsems (or

if there is an issue accessing this), this will return False.
The result is cached in an LRU cache to avoid repeated lookups for the

same paths. In practice, this is going to be used only with very few

hard-coded paths (such as a local queue path or parts of the data

directory).

Unit tests pass.