Rejigger contract between LeaderService and rest of components

[srkukarni]

(If this PR fixes a github issue, please add Fixes #<xyz>.)

Fixes #

(or if this PR is one task of a github issue, please add Master Issue: #<xyz> to link to the master issue.)

Master Issue: #

Motivation

Currently LeaderService exposes a isLeader which is used by the rest of the system components to mean that this worker is leader and can server leader specific commands(updateOnFunctionLeader for example). However isLeader is set prematurely in LeaderService even before the worker has finished all the init steps of becoming a leader.
This pr fixes that and ensure that isLeader is true only if the worker is ready to perform the role of the leader. This in turn simplifies the workflow of other components(like functionmetadata manager and schedulermanager).

Modifications

Describe the modifications you've done.

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)
• The rest endpoints: (yes / no)
• The admin cli options: (yes / no)
• Anything that affects deployment: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation