Limited use trusts

Trusts now have a "remaining_uses" field that track how many times
a trust can still issue a token. It is decremented each time a
trust related authentication occurs. If set to null, trusts can be
used indefinitely to authenticate.

Fix for bug #1250617

Change-Id: I739cd583eea2363724b888e9c2b9b9158a9838c1
Merge "Limited use trusts"