Allow subclasses to override query-templates used

[fabianbuch]

Currently the queries are defined as constants within @JdbcMessageProvider@:

    private static final String QUERY_INSERT = "INSERT INTO %s (%s, %s, %s, %s, %s, %s) VALUES (?, ?, ?, ?, ?, ?)";
    private static final String QUERY_DELETE = "DELETE FROM %s WHERE %s = ?";
    private static final String QUERY_SELECT_BASENAMES = "SELECT DISTINCT %s from %s";
    private static final String QUERY_SELECT_MESSAGES = "SELECT %s,%s,%s,%s,%s FROM %s WHERE %s = ?";

Reading those should be wrapped in protected metods like:

protected String getSelectMessagesQuery() {
  return QUERY_SELECT_MESSAGES;
}

So users could customize the queries by doing the following in a subclass:

@Override
protected String getSelectMessagesQuery() {
  return super.getSelectMessagesQuery() + " WHERE account = 2";
}

[fabianbuch]

Comment of Marc Kannegiesser: I introduced the needed methods:

protected String getSelectMessagesQuery();
protected String getSelectBasenamesQuery();
protected String getInsertMessageQuery();
protected String getDeleteMessagesQuery();

There is still some documentation (WIKI) to be done so ill leave the ticket open for now.
The feature is available from the latest snapshot-build if anyone wants to try... (0.7-SNAPSHOT).