FAB-15426 fix Raft UT flake
FAB-15426 fix Raft UT flake

- use Mutex instead of RWMutex, so that there can be only 1 message
  being sent at a time, so that if we perform some other operations
  when sending a message, i.e. disconnect network, next message will
  deterministically respect the outcome
- add more assertions to make sure messages are being dropped as expected

Change-Id: I3b429086db9c9e46454c758bb6b01a929047c015
Signed-off-by: Jay Guo <91d7eeb83d08be0dc56ea674009c4b53383e15b3@gmail.com>

muralisrini/fabric,stemlending/fabric,stemlending/fabric,manish-sethi/fabric-sidedb,jimthematrix/fabric,muralisrini/fabric,jimthematrix/fabric,manish-sethi/fabric-sidedb,stemlending/fabric,stemlending/fabric,manish-sethi/fabric-sidedb,hyperledger/fabric,binhn/fabric,binhn/fabric,muralisrini/fabric,binhn/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,manish-sethi/fabric,hyperledger/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric