Cleanup: avoid immediate Optional.get()

Cleanup: avoid immediate Optional.get()

This reworks allocateId() to store the id directly, and wrap it in an
Optional when needed, instead of wrapping it and unwrapping it
immediately. The performance gain isn't significant, but this helps
static analysers which check for get()-without-isPresent().

Change-Id: I5155660e6240a8c9d01de0bce78528b80d755e10
Signed-off-by: Stephen Kitt <skitt@redhat.com>