WL#3359: Decouple binary protocol from prepared statements

Choosing to use MySQL prepared statements requires some compromises, such as an 
extra round trip for the "prepare" phase and a disabled query cache.

In contrast, using MySQL's binary protocol for result sets is almost always good 
for performance.  It should be possible to use the binary protocol without 
prepared statements.
 
A client should be able to send a command COM_RESULTS_BINARY, requesting for all 
future result sets to be returned using the binary protocol.  A similar command, 
COM_RESULTS_TEXT, would request that all future result sets use the text protocol. 

(Remark: no need to define a new command for that. We already have
COM_SET_OPTION, currently used to enable/disable multi statements. We have
2^16-2 other possible option values left. We could use two of them for
use_binary_result and use_text_result)