feat(v1.1): Default TTL and grace values

When posting messages, the "ttl" field may be omitted, in which
case the server defaults to the default TTL value configured by
the operator.

Likewise, when creating a claim, the "ttl" and "grace" fields
may be omitted to use the defaults configured for each field.

Change-Id: I38c6307377293b26370de28ffa13db22228c208c
Partially-Implements: blueprint api-v1.1-request-document-changes
Merge "feat(v1.1): Default TTL and grace values"