new_execution incurs unnecessary performance hit

[bsanders1979]

mysql.rb:411-414
...
metadata = res.result_metadata rescue nil

if metadata
columns = res.result_metadata.fetch_fields.collect do |col|
...

I was profiling my code to find bottlenecks and discovered that result_metadata was costing me a lot of CPU time
Line should be: columns = metadata.fetch_fields

Also, seems like metadata/columns/schema should only be calculated once unless there's a good reason why it shouldn't be.

[bsanders1979]

I have re-factored this function such that schema has been converted to an instance variable that is lazily initialized upon the first execution of the prepared statement. The end result is that my engine now runs over 6 times faster and maintains consistent throughput which is a big deal.