WL#1278: Create an ODBC-backed Table Handler

Some users would find it useful to create tables inside MySQL that are actually
tables that exist on other non-MySQL databases. It should be possible to create
a table handler that uses ODBC to expose this data as tables to a MySQL server
through the table handler/storage engine interface.

The problem is that MySQL uses FRM file for every table,
and field list with their data types,lengths, flags is
loaded from this file. Adding this handler required moving
this functionality into handler level, i.e we should fetch
table strucuture from the remote server every time we
execute a query for a remote table. It might be dramatically
slow.

Another problem is transaction handling. What should we do
if MySQL rallbacks or commits a transaction? Should we also 
do the same for a remote table?
What if remote transaction fails on commit? Should we
also rallback local transaction?