Refresh token support

The refresh token may now be used to obtain a new access token if
needed. The refresh token can be used only once.

Sample request:
POST /v1/auth/token?grant_type=refresh_token&refresh_token=<the_token>

The response will be similar with the one if authroization code is used.

Introduced expiration for refresh tokens.
The migration deletes existing refresh tokens as they do not have a
valide expiration information set. Probably noone should notice that
because client is not using refresh tokens currently.

Change-Id: Ie0924888f66ca956caa43b04b8501e6fa8e9371e
Merge "Refresh token support"