[FAB-2944]: Make leadership callback synchronous
[FAB-2944]: Make leadership callback synchronous

In order to make leader election test consistent and not to fail this
commit makes switching leadership state synchronous.

Change-Id: Ie04155bd089f0e7e8553bca9df33e14e0d332817
Signed-off-by: Artem Barger <6d127d713cb520aa658cf81ac2373d74ffc1958c@il.ibm.com>

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