odbc_write_table: describe parameters only if column data is coherent

[detule]

Fixes #586

When writing out our oracle specific implementation of odbcConnectionColumns we snooped on how the OEM driver implements the SQLColumns ODBC API call. As a result we inherited some issues - for example, the OEM query always returns 0 for the DATA_TYPE field in the result ( we return NA ).

This is a problem, because if the output of odbcConnectionColumns/SQLColumns is available, our dbWriteTable implementation will attempt to use it as a performance optimization / to reduce the number of round-trips to the server to describe each of the parameters of the INSERT query.

With this patch, we attempt to optimize ( pre-describe the parameters ) only if we have a coherent result from the odbcConnectionColumns query. Coherent is defined as data_type is not NA and the number of rows ( each row describing an individual parameter ) is equal to the number of parameters in the INSERT query.

Beyond this patch, we should snoop on the SIMBA driver as well to see if it has a better / more complete way of querying column types of existing tables, and use this to update the ORACLE specific implementation.

Finally, probably also worth thinking about

• Adding ORACLE unit testing infrastructure.
• Adding MSSQL / FreeTDS unit testing infrastructure as for that DB/Driver pairing the parameter description is not just an optimization, it is a necessity. It is important we don't disrupt how the parameter description works/is used in that case.