Qbft RPCs should be disabled after starting with or switching to use validator contract

[siladu]

PR description

Qbft RPCs should be disabled after starting with or switching to use validator contract
Signer metrics and getValidatorsByBlock... should remain enabled.

Fixed Issue(s)

Related to #2795

Changelog

• I thought about the changelog and included a changelog update if required.

[siladu]

Just discovered a bug when transitioning back to blockheader mode, e.g. with this transitions config

"transitions": {
      "qbft": [
        {
          "block": 2,
          "validatorselectionmode": "contract",
          "validatorcontractaddress": "0x0000000000000000000000000000000000008888"
        },
        {
          "block": 4,
          "validatorselectionmode": "blockheader"
        }
      ]
    }

2021-10-06 21:10:01.007+02:00 | pool-8-thread-1 | ERROR | EventMultiplexer | State machine threw exception while processing event \{BlockTimerExpiry{Round Identifier=ConsensusRoundIdentifier{Sequence=4, Round=0}}\}
java.lang.IllegalStateException: Bft requires a vote provider
	at com.google.common.base.Preconditions.checkState(Preconditions.java:510)
	at org.hyperledger.besu.consensus.common.bft.blockcreation.BftBlockCreatorFactory.createExtraData(BftBlockCreatorFactory.java:108)
	at org.hyperledger.besu.consensus.qbft.blockcreation.QbftBlockCreatorFactory.createExtraData(QbftBlockCreatorFactory.java:93)
	at org.hyperledger.besu.consensus.common.bft.blockcreation.BftBlockCreatorFactory.lambda$create$1(BftBlockCreatorFactory.java:82)
	at org.hyperledger.besu.ethereum.blockcreation.AbstractBlockCreator.createBlock(AbstractBlockCreator.java:182)
	at org.hyperledger.besu.ethereum.blockcreation.AbstractBlockCreator.createBlock(AbstractBlockCreator.java:126)
	at org.hyperledger.besu.consensus.qbft.statemachine.QbftRound.createAndSendProposalMessage(QbftRound.java:100)
	at org.hyperledger.besu.consensus.qbft.statemachine.QbftBlockHeightManager.handleBlockTimerExpiry(QbftBlockHeightManager.java:125)
	at org.hyperledger.besu.consensus.common.bft.statemachine.BaseBftController.handleBlockTimerExpiry(BaseBftController.java:140)
	at org.hyperledger.besu.consensus.common.bft.EventMultiplexer.handleBftEvent(EventMultiplexer.java:54)
	at java.base/java.util.Optional.ifPresent(Optional.java:183)
	at org.hyperledger.besu.consensus.common.bft.BftProcessor.run(BftProcessor.java:60)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:829)

[siladu]

Split out the transition bug onto another issue #2868

[jframe]

LGTM