Fixed name sequence of prepared statements bug

Author: marat-gainullin

src/main/java/com/github/pgasync/PgConnection.java

@@ -127,7 +127,7 @@ private String next() {
         }
     }
 
-    private static final NameSequence preparedStatementNames = new NameSequence("s-");
+    private final NameSequence preparedStatementNames = new NameSequence("s-");
 
     private final ProtocolStream stream;
     private final DataConverter dataConverter;