Use strict lookup for op permission refresh - Resolves BUKKIT-4698

The Issue:

Adding or removing operators was incorrectly using loose player lookup methods
which could incidently cause permission refreshes on the wrong player.

Add/Remove op operations are exact name match only and the permission refresh should
behave the same way.

PR Breakdown:
Changed getPlayer() to getPlayerExact()

JIRA Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-4698