Refactor path splitting and validation.

The account, container, and object servers all do the same thing at
the top of GET/PUT/etc.: they split the path, validate that the first
two components are a good (device, partition) pair, and return a 400
if they're not. The object server already had a module-local helper
function for this, but the account and container servers just had
duplicate boilerplate code. Now it's all in one common helper.

Change-Id: I9a20d37fc9e1a68b10149a7aa78cb9691fc04ea9