add transaction store

The idea is to provide a global and a per-user transaction log.

The global transaction log:

• it is shared for all users.
• it supports getting a transaction, but you must know the exact id.
• ids have the form scope/key, where the scope is used for acl.
• it supports pre-reserving a unique key and adding a new entry.

The per-user transaction log:

• it supports getting all transactions in one scope and adding a new transaction.
• the payload is encrypted for the user.

Both transaction logs have a ttl, after which entries expire.

TODO:

• obfuscate creation date (maybe?)