Force issuing heartbeats when appending to Raft log

Summary:
Locally run HA feature benchmark:

```
duration: 20.66
executed_writes: 150007
write_per_second: 7527.89
```

Reviewers: msantl

Reviewed By: msantl

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D1809

src/raft/raft_server.cpp

@@ -246,6 +246,13 @@ void RaftServer::AppendToLog(const tx::TransactionId &tx_id,
   ++last_applied_;
   disk_storage_.Put(LogEntryKey(log_size), SerializeLogEntry(new_entry));
   disk_storage_.Put(kLogSizeKey, std::to_string(log_size + 1));
+
+  // Force issuing heartbeats
+  TimePoint now = Clock::now();
+  for (auto &peer_heartbeat : next_heartbeat_)
+    peer_heartbeat = now;
+
+  state_changed_.notify_all();
 }
 
 void RaftServer::Emplace(const database::StateDelta &delta) {

tests/feature_benchmark/ha/raft.json

@@ -1,6 +1,6 @@
 {
   "election_timeout_min": 100,
   "election_timeout_max": 300,
-  "heartbeat_interval": 1,
+  "heartbeat_interval": 50,
   "replicate_timeout": 100
 }

tests/feature_benchmark/ha/runner.sh

@@ -20,7 +20,7 @@ fi
 RESULTS="$DIR/.apollo_measurements"
 
 # Benchmark parameters
-NODES=15000
+NODES=150000
 
 ## Startup
 declare -a HA_PIDS