Allow multiple active TAAs

Story

As an organization using an Indy Network, I need to adopt the Transaction Author Agreement at the time my legal team approves it, rather than coordinating closely with the network administrators for when to cut-over to a new agreement.

Current TAA flow

• Network governance adopts a TAA, and then publishes it on a web site.
• Users of the network can review the TAA.
• After the advisory period has expired, it is published to the ledger and immediately enforced and the old agreement is immediately retired.

Problems

• Users of the network have to wait to adopt the new TAA until the advisory period has expired and it is on the ledger even if they have already accepted it.
• Users of the network have to immediately switch versions of the TAA in their software at the time the new TAA is on the ledger.
• Users of the network can't store their acceptance in their software configuration until the TAA is on the ledger and the official hash is available.

Revised TAA flow

• Network governance adopts a TAA, and it enters the public advisory period.
• It is immediately put on the ledger.
• When the new agreement's public advisory period has ended and the old agreement is no longer acceptable, network governance marks the old agreement as retired.

Acceptance Criteria

• When a new TAA is put on the ledger, old agreements are not automatically rejected. Marking a TAA as "retired" is done in a separate transaction from adding a new TAA.
• A transaction that includes acceptance of any non-retired TAA should be accepted by the ledger.
• A query to the ledger for the TAA which does not specify a specific version should always return the newest TAA.
• Documentation is updated.

Notes

• The number of acceptable TAAs is determined by the network admin based on when agreements are marked as retired. It is expected that only one TAA will be active most of the time, but during a transition period two TAAs may be active. It is expected that there will never be more than two TAAs active at a time. However, we don't need to enforce that on the ledger.
• The current logic that a TAA can be accepted the day before it gets on the ledger (due to clock skew) should be preserved.