[2.2] Update coordination strategy of FlatSearchCommandHandler

[Skoucail]

Add MRCluster_MastersOnly to the coordination strategy of the FlatSearchCommandHandler This fixes #326

[Skoucail]

@rafie Can you help me? I have no clue why that 1 single test fails on the build server because of my PR

[rafie]

Hello @Skoucail, this is a flaky test, just needs a re-run. However, please note we no longer accept PRs on the RSCoordinator repo (as RSCoordinator is now integrated into the RediSearch repo). Instead, please open a PR on RediSearch/master, and modify files under the coord directory. Thanks!

[Skoucail]

@rafie Oh okee, didnt know this repo is depricated now. I did a test with the RediSearch 2.2 branch. Same issue. So i recreated the pull request over in RediSearch: https://github.com/RediSearch/RediSearch/pull/2404

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