Update router cache on new function version

[soamvasani]

Addresses issue #51. Makes versioning in poolmgr explicit, so its
cache needs no invalidation on version update. Functions in poolmgr
are always cached by name and UID.

Router now updates implictly versioned routes with the latest version
of a function.

This means that users will see requests to implicitly-versioned routes
go to the latest version of a function within 3 seconds. (Those 3 sec
will go away when we use a real watch instead of polling the
controller.)

There's no change in behaviour for routes that explictly specify a
function UID.