perf: cache result of parsing server_version

Save version number after parsing it from string.
No need to recalculate it again.

Backend does not always send server_version_num, thus the driver falls back to string parsing.
See discussion on hackers list: http://www.postgresql.org/message-id/CAMsr+YFt1NcjseExt_Ov+frk0Jzb0-DKqYKt8ALzVEXHBM0jKg@mail.gmail.com

This improves Statement.setObject(int, UUID) which ckecks for server version.

closes #464

pgjdbc/src/main/java/org/postgresql/core/v3/ProtocolConnectionImpl.java

@@ -57,7 +57,7 @@ public String getServerVersion() {
     public int getServerVersionNum() {
         if (serverVersionNum != 0)
             return serverVersionNum;
-        return Utils.parseServerVersionStr(serverVersion);
+        return serverVersionNum = Utils.parseServerVersionStr(serverVersion);
     }
 
     public synchronized boolean getStandardConformingStrings()