Sybase/MSSQL: Fix for repeated EED messages

This pull request fixes repeated EED server messages causing error handling to break subsequent queries. An error like this can be provoked by issuing a query as follows:

```sql
select not_the_table_name.field1, not_the_table_name.field2 from table_name
```

This pull request adds an integration test to verify this. If this test is run with the implementation existing on the master branch, the effect can nicely be seen:

```
F unittest.TestError(test= net.xp_framework.unittest.rdbms.integration.SybaseIntegrationTest::repeated_extend_errors, time= 0.027 seconds) {
  Exception rdbms.SQLStatementFailedException (errorcode 0: Unexpected token 0x5F (number 0)) {
    select 1
  }
```

## Contents

* Fix repeated EED messages leading to "Unknown token" messages
* Enhance test to actually fail on master branch
* Empty messages before logging in and before issuing queries
* No longer empty messages on every read
* QA: Add apidoc link to Sybase TDS v5 spec
* Merge all messages into the exception cause

See xp-framework/xp-framework#360