feat: Enroll drop revoked clients

[murali-shris]

Closes https://github.com/atsign-foundation/at_server/issues/1927 - What I did

• When an enrollment is revoked and if that enrollment has any active client, close the connection to the client on the server - How I did it
• Implemented a method _dropRevokedClientConnection which drop connection on the server when enrollment is revoked
  • If the connection to drop is not the current connection, directly close the connection without any warning
  • if the connection to drop is current connection and force flag is set..send a message to client in the response and drop the connection after 10 seconds.
• Modified enroll verb functional tests to check for inbound connection count before and after enroll:revoke - How to verify it
• unit and functional tests should pass

[murali-shris]

Resolved the functional test failure for enroll:revoke:force working on functional test for revoking enrollment and verify using stats verb whether connection got closed