Improve JobOperator by reducing its scope to job operations only

[fmbenhassine]

As of v5.2, the JobOperator interface is exposing functionality that is out of its initial scope (ie operating batch jobs like start/stop/restart, etc).

Several methods like getJobInstance, getExecutions and similar methods have duplicate implementations in JobRepository and JobExplorer which is not ideal from a maintenance perspective . Moreover, some methods deal with job parameters conversion from string literals to domain objects which is not correct in a high-level API like JobOperator (those could be part of CommandLineJobRunner for example).

Those methods should be deprecated in v6.0 and removed in v6.2 or later.