fs: factor out kernel ID management in a separate type

Currently, fs/ provides a hard-coded, in-memory ID management
strategy, which makes restarting servers impossible (see also #618).

As a first step to addressing this, refactor ID management
functionality into a separate type. The API surface is still
extensive, so don't export this just yet.

Change-Id: If2aebf493f0a59e5742e963c8f6bedc66a6a6964

Author: hanwen