Add methods to get and list Cluster databases (#120)

## What is the goal of this PR?

We added new service methods to Cluster that retrieve Cluster databases, including their replica info. Although this was possible before with `database_replicas`, this method was not well aligned with what Core does. Using the terms `get` and `all` allows us to establish a more Core-like API.

## What are the changes implemented in this PR?

- Add `database_all` and `database_get` methods to Cluster service

Author: Alex Walker

protobuf/cluster/database.proto

@@ -23,22 +23,31 @@ option java_outer_classname = "DatabaseProto";
 package grakn.protocol.cluster;
 
 message Database {
+    string name = 1;
+    repeated Replica replicas = 2;
 
     message Replica {
         string address = 1;
-        string database = 2;
-        bool primary = 3;
-        bool preferred_secondary = 4;
-        int64 term = 5;
+        bool primary = 2;
+        bool preferred_secondary = 3;
+        int64 term = 4;
     }
 
-    message Replicas {
+    message Get {
         message Req {
-            string database = 1;
+            string name = 1;
         }
 
         message Res {
-            repeated Replica replicas = 1;
+            Database database = 1;
+        }
+    }
+
+    message All {
+        message Req {}
+
+        message Res {
+            repeated Database databases = 1;
         }
     }
 }

protobuf/cluster/grakn_cluster.proto

@@ -27,5 +27,7 @@ package grakn.protocol.cluster;
 
 service GraknCluster {
     rpc cluster_servers (Cluster.Servers.Req) returns (Cluster.Servers.Res);
-    rpc database_replicas (Database.Replicas.Req) returns (Database.Replicas.Res);
+
+    rpc database_get (Database.Get.Req) returns (Database.Get.Res);
+    rpc database_all (Database.All.Req) returns (Database.All.Res);
 }