[WIP][TEZ-3904] An API to update tokens for Tez AM and the DAG

[beltran]

This PR has ended up bigger than what I expected so I wanted to ask for some feedback before going forward. This is what is does mainly:

• Adds updateAMCredentials and updateDAGCredentials to TezClient. This is not the final API but this functions will be useful anyway to send the credentials
• Add two corresponding functions to the DAGClientAMProtocol
• Add a SystemEventHandler in the runtime internals to process the UpdateCredentialsEvents
• Add the UpdateCredentialsEvent event and some transitions to the TaskImpl's state machine that represent the credentials being updated.

TODO:

• Add more tests. Will do after the feedback
• Log to history the credentials change?
• I've added some logic to renew the session credentials, these credentials should be updated as well?