Perf: Cache delegates for non query and xml reader calls

[Wraith2]

When using ExecuteNonQueryAsync and ExecuteXmlReaderAsync begin and end delegates are passed to Task<T>.Factory.FromApm and because these are instance methods they cannot be cached meaning we have two delegates allocated per call. ExecuteNonQueryAsync is a very commonly used method so even small improvements here can have a large impact overall.

I followed the general pattern introduced and used in ExecuteReaderAsync. I created static versions of the begin and end functions needed for each of the FromApm calls and made sure that the instance was passed as a state variable, the static functions simply unpack the instance from state and call the correct method on the instance. The static delegates are then cached as static readonly and re-used every time a call is made.

Note that the netfx version of ExecuteReaderAsync has not been altered because it will be dealt with by #1084

[Wraith2]

I'm closing this because there is a cleaner and lower impact way to do this once #1084 has been merged.