Fix float64 de/serialization

Hi Ryan - Discovered a bug while trying to insert float64 into a FLOAT column.

Turns out both serialization and deserialization is broken, so I wrote some tests and fixed the issue by using math.Float64bits()/Float64frombits() for converting to/from uint32 after/before reading/writing the raw bytes.

Have confirmed the fix by round-tripping through a voltdb instance :)