Summary:
Created a new integration test for Raft protocol.
The tests iterates through the Raft cluster and does the following:
* kill machine `X`
* execute a query
* bring `X` back to life
The first step is to insert a vertex in the cluster, and last step is to check
if the cluster has all the data.
I also edited some of the raft core files because this test surafaced some bugs.
The `tester` binary is a hacked version of the HA client and so are the parts in
the code that refuse to execute a query is the machine is not in `Leader` mode.o
Those parts will go away once we have a proper HA client.
I've run the `runner.py` for a while (215 times)
```
while ./runner.py &> log.txt; do echo -n "."; done
```
and it didn't break.
Reviewers: ipaljak, mferencevic
Reviewed By: ipaljak
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D1788
