Kobe API: Retrieve only actual validator history entries (#20)

# Problem
- We returned default valdiator history entry too right now

# Solution
- Fix to return actual validator history entry

Author: aoikurokawa

README.md

@@ -287,7 +287,7 @@ cargo build --release
 cargo r --bin kobe-api -- \
     --ip 127.0.0.1 \
     --port 8080 \
-    --mongo-connection-uri "" \
+    --mongo-connection-uri "mongodb://<username>:<password>@localhost:27017" \
     --mongo-db-name validators \
     --sentry-api-url "" \
     --rpc-url "https://api.testnet.solana.com"

api/src/resolvers/query_resolver.rs

@@ -842,6 +842,7 @@ impl QueryResolver {
                 .arr
                 .iter()
                 .map(ValidatorHistoryEntryResponse::from_validator_history_entry)
+                .filter(|history| history.epoch.ne(&u16::MAX))
                 .collect(),
         };