Enabling the management of validators.

[0xmenna]

In order for validators to manage keys, it has been integrated the session pallet into the runtime.
It allows validators to have an address that points to its session keys, used for block production and finality.
Each session takes 600 blocks which corresponds roughly to about 1 hour.

It has also been added the validatorManager pallet to solve the issue of the genesis config of the session pallet. In fact it would not work out of the box because the genesis configuration did not contain the right infos. For this reason the node won't compile correctly because there is no genesis configuration. To upgrade the runtime just compile the runtime to the wasm blob.

Configuration:
To configure correctly the session pallet after the runtime upgrade it's necessary to use the validatorManager pallet.

1. Call validatorManager.sessionConfig(keys) as sudo providing the validators keys (first two parameters can easily be the account ids that correspond to the aura keys, third parameter is aura key and fourth grandpa key).
   WARNING: It is fundamental to dispatch this call by the time the session begins which happens at a multiple of 600 blocks.

Registration and Retirement of validators:

• Call deregisterValidators and registerValidators as sudo to respectively delete and/or add validators from the set.
  The changes will take place two sessions after the extrinsic execution (it will be queued at the next session, so it must wait one more session in order for it to take place).

[Polkaverse]

Changes looks good. Not tested the code functionally.
We would need to write unit-testing testing and benchmarking for the new extrinsics.