MySQLnd: Clean up and optimize mysqlnd result set handling

MySQLnd: Clean up and optimize mysqlnd result set handling

This is a larger overhaul of the mysqlnd result set infrastructure:

 * Drop support for two different types of buffered results sets
   ("c" and "zval"). Possibly these made sense at some earlier
   time, but now (with minor adjustments) one option is strictly
   worse than the other. Buffered result sets already buffer the
   full row packets, from which zvals can be decoded. The "zval"
   style additionally also buffered the decoded zvals. As result
   sets, even buffered ones, are generally only traversed once,
   this just ends up wasting memory. Now, a potentially useful
   variation here would be to buffer the decoded zvals instead of
   the row packets, but that's not what the code was doing.
 * To make it really strictly better, pre-allocate the zval row
   buffer and reuse it for all rows. Previously the "c" style always
   allocated a new buffer for each row.
 * The fetch_row API now provides a populated zval[]. The task of
   populating an array is deferred to fetch_row_into, which also
   avoids duplicating this code in multiple places. The fetch_row_c
   API is also implemented on top of fetch_row now, rather than
   duplicating large parts of the code.
 * The row fetching code for prepared statements and normal result
   sets has been mostly merged. These already used the same
   infrastructure, but prepared statements used separate row
   fetching functions that were nearly the same as the normal ones.
   This requires passing the stmt into the result set, rather than
   just a flag. The only part that remains separate is reading of
   unbuffered results in the presence of PS cursors.