Change user_id into string

Cloud users are going to be represented by strings, probably email
address. Integer prefix is used to avoid collisions between
pairing/anonymous and local users.

BUG=25768507

Change-Id: I27249c0b98f919e9527498be74ddaa82218b4041
Reviewed-on: https://weave-review.googlesource.com/2063
Reviewed-by: Alex Vakulenko <avakulenko@google.com>