Changed behavior of notify::setLastRead custom operation - right now passed...

Changed behavior of notify::setLastRead custom operation - right now passed date exceeding head block time is implicitly corrected to head block time.
Also there was relaxed syntax: `date' field is optional (can have also null value) what also leads to saving head block time as lastRead time specific to the account related to operation.
Prepared initial set of trivial mock tests (to be continued).