ByteaTextToByteArray conversion optimization

Hi Francisco,

I have my own GIS project that mostly deals with reading of geospatial data.
By doing some profiling of my application I've found out that the function ByteaTextToByteArray in the BasicBackendToNativeTypeConverter class takes too much time to convert data from hex format.
Then I tried to find a way to speed up the conversion.
It seems that current implementation is not well optimized:

  	     MemoryStream ms = new MemoryStream(); <-  is unnecessary object
            if (BackendData.StartsWith("\\x"))
            {
                // PostgreSQL 8.5+'s bytea_output=hex format
                for (byteAPosition = 2; byteAPosition < byteAStringLength; byteAPosition += 2)
                {
                    byte value = Convert.ToByte(BackendData.Substring(byteAPosition, 2), 16);     <- substring and byte conversion is slow


Master branch: 46117 ms           12566  - 3.67
Size: 483 MB
Rows: 2260348

Patched branch: 10639 ms          - 4.33 times