[M-01] Approval Periods Are Implicitly Restricted

[brozorec]

close #35

The audit report suggests two ways of addressing this issue:

Consider allowing accounts to specify approvals valid for longer than the maximum TTL value by limiting the TTL extension period to the maximum possible value. ...

and

Alternatively, consider using the permanent storage for storing allowances.

We acknowledge the reasoning behind this suggestions, but decided to keep the current design of implicitly restricted approvals. The main reason is consistency with SAC (Stellar Asset Contract), where temporary storage is used for this data and one can't set a longer TTL than the max allowed. However, we introduce an explicit check/error for the case when live_until_ledger > max_live_until_ledger and explain this limitation in the docs.