Validate rollapps' constrains in CreateSequencer

[liorzilp]

Overview

When creating a sequencer it is attached to a specific rollapp instance (identified by rollappID), and this rollapp should be already existing. A rollapp also has a limit of maximum sequencers that could be attached and might have a list of permissioned sequencers. Both needed to be checked when creating a new sequencer.

Basic flow

On CreateSequencer

1. Retrieve rollapp object from the rollapp keeper
    - if not exists return ErrUnknownRollappId
2. If PermissionedAddresses list isn't empty:
    - check if the sequencer is in the list, if not return ErrSequencerNotPermissioned
3. Check that number of rollapps attached to this rollapp does not exceed MaxSequencers:
    - if not(SequencersByRollapp(rollappId)<rollapp.MaxSequencers) return ErrMaxSequencersLimit

Tests

 1. ErrUnknownRollappId
 2. ErrSequencerNotPermissioned
 3. ErrMaxSequencersLimit